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In this case most of the positive tests are false positives and the further testing that is required to determine wether the test is a false or true positive usually requires invasive testing such as operative biopsy purchase super p-force oral jelly 160 mg free shipping erectile dysfunction wellbutrin xl. Finally order genuine super p-force oral jelly erectile dysfunction medicine by ranbaxy, 312 Essential Evidence-Based Medicine Table 28 generic super p-force oral jelly 160 mg without a prescription erectile dysfunction treatment karachi. Criteria for a valid screening test (1) Burden of suffering The disease must be relatively common buy 160 mg super p-force oral jelly overnight delivery erectile dysfunction is often associated with quizlet. Criteria for screening There are five criteria that must be fulfilled before a test should be used as a screening test. Following these rules will prevent the abuses of screening tests that occurred in the 1950s and 1960s and which continue today. The disease must impose a significant burden of suffering on the population to be screened. This means either that the disease is common or that it results in serious or catastrophic disability. This disability may result in loss of productive employment, patient discomfort or dissatisfaction, as well as passing the disease on to others. It also means that it will cost someone a lot of money to care for persons with the disease. The hope is to reduce this cost both in human suffer- ing and in dollars by treating at an earlier stage of disease and preventing com- plications or early death. This depends on well-designed studies of harm or risk to tell which diseases are likely to be encountered in a significant portion of the population in order to decide that screening for them is needed. For example, it would be unreasonable to screen the population of all 20-year- old women for breast cancer with yearly mammography. The risk of disease is Screening tests 313 so low in this population that even a miniscule risk of increased cancer asso- ciated with the radiation from the examination may cause more cancers than the test would detect. Similarly, the prevalence of cancer in this population is so low that the likelihood a positive test would be cancer is very low and there will be many more false positives than true positives. The screening test must be a good one and must accurately detect disease in the population of people who are in the presymptomatic phase of disease. It should also reliably exclude disease in the population without disease or have high specificity. Of the two, we want the sensitivity to be perfect or almost perfect so that we can identify all patients with the disease. We’d like the specificity to be extremely high so that only a few peo- ple without disease are mislabeled leading to a high positive predictive value. This usually means that a reasonable confirmatory test must be available that will more accurately discriminate between those people with a positive screen- ing test who do and don’t have the disease. It should be relatively comfortable, not very painful, should not cause serious side effects, and also be reasonably priced. A screening test may be unacceptable if it produces too many false positives since those people will be falsely labeled as having the disease, a circumstance which could lead to psychological trauma, anxiety, insurance or employment discrimination, or social conflicts. Several studies have found significant increases in anxiety that interferes with life activities in persons who were falsely labeled as having disease on a screening test. This is an especially serious issue with genetic tests in which a positive test does not mean the disease will express itself, but only that a person has the gene for the disease. For screening tests, most people will tolerate only a low level of discomfort either from the test procedure itself or from the paperwork involved in getting the test done. People would much rather have their blood pressure taken to screen for hypertension than have a colonoscopy to look for early signs of colon cancer. Finally, people are more willing to have a test performed to detect disease when they are symptomatic than when they are well. If the test is too complex such as screening colonoscopy for colon cancer, most people would not be willing to have it done. A test that is very uncomfortable such as a digital rectal exam for prostate or rectal cancer, may be refused by a large proportion of patients. Both examples also require more complex logistics such as individual examin- ing rooms and sedation for the colonoscopy than a screening test such as blood pressure measurement. Screening tests must also be well advertised so that peo- ple will know why and how to have the test done. Pitfalls in the screening process Simply diagnosing the disease at an earlier stage is not helpful unless the progno- sis is better if treatment is begun at that earlier stage of the illness. The treatment must be acceptable and more effective before people will be willing to accept treatment at an asymptomatic stage of illness. Why should someone take a drug for hypertension if they have no signs or symptoms of the disease when that drug can cause significant side effects and must be taken for a lifetime? During the 1960s and 1970s, some lung cancers were detected at an earlier stage by routine screening chest x-rays. However, immediate treatment of these cancers did not result in increased survival and caused increased patient suffer- ing due to serious side effects of the surgery and chemotherapeutic drugs. There- fore, even though cancers were detected at an earlier stage, mortality was the same. The validity of a screening test can be determined from the evidence in the literature. Screening tests must balance the need to learn something about a patient, the diagnostic yield, with the ability to actively and effectively intervene in the disease process at an earlier stage. Lead-time bias results in over- optimistic results of the screening test in the clinical study. The patients seem to live longer but this is only because their disease is detected earlier. In this case, the total time from onset of illness to death is the same in the group of patients who were screened and treated early compared with the unscreened group. The lead time is the time from diagnosis of disease by screening test to the appear- ance of symptoms. The time from appearance of symptoms to death is the same whether the disease was detected by the screening test or not. The total life span of the screened patient is no different from that of the unscreened patient. The time between early diagnosis with the screening test and appearance of symp- toms, the lead time, will now be spent undergoing treatment (Fig. This Screening tests 315 Onset of Fig. Onset of Asymptomatic Death Symptoms disease No screening Survival Time Screened but early treatment not effective Apparent Survival Time (lead-time bias) Screened and early treatment is effective (No lead- time bias, time Prolonged Survival Time effectiveness) could be very uncomfortable due to the side effects of treatment or even dan- gerous if treatment can result in serious morbidity or death of the patient. Patients are not randomized and the spectrum of disease may be very different in the screened group when compared to the unscreened group. A disease that is indolent and slowly progressive is more likely to be detected than one that is rapidly progressive and quickly fatal.

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Wetland users do not need to become disease experts but communication and awareness raising programmes should aim to increase motivation to become engaged and ‘do the right thing’ buy 160 mg super p-force oral jelly with amex impotence ka ilaj, with respect to disease management 160mg super p-force oral jelly overnight delivery outcome erectile dysfunction without treatment. This will likely only come from becoming informed about the problem buy cheap super p-force oral jelly 160 mg line erectile dysfunction doctors in utah, understanding the issues and implications 160mg super p-force oral jelly fast delivery erectile dysfunction medication for diabetes, and participating in the solutions. Developing capacity to undertake disease management may involve formal education and training of key personnel e. Communication networks of key wetland stakeholders, including disease control authorities, should be established in ‘peacetime’ to facilitate rapid disease control responses should the need arise. This Manual aims to provide some of the information as a foundation for communication and public awareness programmes. The concept of ‘One World One Health’ has arisen due to the appreciation of the fundamental connectivity in health of humans, domestic livestock and wildlife. Embracing an ecosystem approach to health in wetlands involves recognising the dependence of health and well-being on ‘healthy wetlands’ which can only be achieved through wise use, most often at a landscape and/or catchment scale. If wetland stakeholders understand both the impacts of diseases and how to prevent and control them, they will feel motivated and empowered to take action. Stakeholder understanding must be built through effective communications or training but action will also be influenced by capacity to respond. To view disease management as separate to other forms of land and wildlife management ensures that opportunities for good disease prevention will be missed. Therefore, integrating disease management into wetland management means putting disease consideration at the heart of the wetland management planning process. Effective management of any disease is dependent on a good understanding of its epidemiology and the ecology of host populations. The dynamics of disease in wildlife populations can be highly complex, and disease management interventions can have unpredictable outcomes. Invasive alien species and novel pathogens and parasites have many parallels in their biology, the risks they pose, and in the measures required to prevent their establishment and control. Prevention of their introduction is preferable to subsequent control, and wetland management practices aimed at prevention of any of these can provide additional benefits and protection from all. In: Global biodiversity mechanisms: a thematic review of recent developments and future evidence needs. Rinderpest and peste des petits ruminants: Virus plagues of large and small ruminants (Biology of Animal Infections). Healthy wetlands, healthy people: a review of wetlands and human health interactions. Secretariat of the Ramsar Convention on Wetlands, Gland, Switzerland, & The World Health Organization, Geneva, Switzerland. Guidance on how to incorporate disease management into management plans for wetlands. Case studies: descriptions and photos of wetland managers’ experiences responding to disease problems. Common Eider Somateria mollissima mortality in the spring and winter of 1999/2000 in the Wadden Sea. Snail fever integrated control and prevention project in Tongxing Village of Wucheng Township, Yongxiu County of Jiangxi Province, P. Training for live wild bird avian influenza surveillance in the Dagona Wetlands of Northern Nigeria However, a risk assessment approach can be used to quantify and/or qualify risks and so help the wetland manager to identify an appropriate course of management actions. Understanding diseases of wildlife and domestic stock, and their public health implications, within wetlands requires a multidisciplinary approach. Advisory groups reflecting a diverse range of knowledge and understanding for specific or general issues, can significantly improve risk assessments and advise on best courses of actions that safeguard both livestock interests and wildlife protection appreciating that these may sometimes seem to be at odds. This is illustrated by the use of advisory groups to deal with highly pathogenic avian influenza H5N1 where it is important to appreciate human health issues, poultry trade and economic issues, alongside factors relating to wildlife such as the behaviour and movements of migratory birds. To ensure the principles and specific actions for disease management and control are embedded within the management practices at a wetland site, they should be written into management plans and updated, as and when, disease risks change. Disease control in endemic disease situations depends upon engagement of all key stakeholders together with their sustained participation and cooperation. Disease control in outbreak situations is similarly dependent on stakeholder engagement but also requires preparedness for a rapid response. Appropriate contingency planning helps to reduce response times and promotes the likelihood of an effective resolution. It is sensible to build a degree of flexibility into contingency plans as unexpected local conditions may be important in determining the action to take. Following implementation of contingency plans, they should be critically reviewed and updated. Clearly defined roles and responsibilities are required to ensure effective management which can deliver a range of benefits to stakeholders. Risk assessments are valuable tools for animal health planning and serve to identify problems/hazards and their likely impact thus guiding wetland management practices. Good local, national and regional surveillance data are needed for robust risk assessments. Multidisciplinary advisory groups provide a broad range of benefits for disease prevention and control. Their role is to review epidemiological and other disease control information, inputting to the activation of agreed contingency plans and advising the appropriate decision makers on future contingency planning. Contingency plans aim to consider possible emergency disease management scenarios and to integrate rapid cost effective response actions that allow the disease to be prevented and/or controlled. Risk assessments should be based on the best available data, which may be quantitative or qualitative. Quantitative assessment of risks associated with wildlife diseases is often difficult due to complex disease dynamics and absence of robust biological data. Qualitative assessments of risk are more usual, within which, risks may be described as “extreme”, “high”, “medium” or “low” or a simple scoring system may be employed. Risk assessments should be revised in the light of new data or changing circumstances. Site assessment: site-specific Framework (Ramsar information on stressor and Convention Secretariat environment. Problem or hazard Risk management/risk reduction identification Manage inputs/alter practices. Useful key information relating to potential disease problems includes: Species known or likely to be affected. Human activities contributing to potential problems or hazards in and around the site (e.

Creation of a safety culture: Reducing workplace injuries in a rural hospital setting buy generic super p-force oral jelly 160 mg on line erectile dysfunction treatment photos. Reducing liver cancer disparities: A community-based hepatitis-B preven- tion program for Asian-American communities buy super p-force oral jelly with visa impotence at 40. Knowledge about hepatitis B and predictors of hepatitis B vaccination among Vietnamese American college students best super p-force oral jelly 160mg pump for erectile dysfunction. A randomized intervention trial to reduce the lending of used injection equipment among injection drug users infected with hepatitis C buy super p-force oral jelly 160 mg overnight delivery erectile dysfunction new zealand. Hepatitis and Liver Cancer: A National Strategy for Prevention and Control of Hepatitis B and C http://www. Why we should routinely screen Asian Ameri- can adults for hepatitis B: A cross-sectional study of Asians in California. Secondary syringe exchange among users of 23 California syringe exchange programs. Risk perceptions and barriers to hepatitis B screening and vaccination among Vietnamese immigrants. Knowledge, attitudes, and behaviors of hepatitis B screening and vaccination and liver cancer risks among Vietnamese Americans. Knowledge, attitudes, and behaviors of Chinese hepatitis B screening and vaccination. Awareness and use of hepatitis B vaccine among homosexual male clients of a Boston community health center. A comparison of trends in the in- cidence of hepatocellular carcinoma and intrahepatic cholangiocarcinoma in the United States. The perceptions and aspirations illicit drug users hold toward health care staff and the care they receive. Gender differences in syringeGender differences in syringe exchange program use in Oslo, Norway. Hepatitis B vaccina- tion in adolescents: Knowledge, perceived risk, and compliance. Awareness and utilization of the hepatitis B vaccine among young men in the Ann Arbor area who have sex with men. Hepatitis and Liver Cancer: A National Strategy for Prevention and Control of Hepatitis B and C http://www. The social production of hepatitis C risk among injecting drug users: A qualitative synthesis. Hepatitis C and its risk management among drug injectors in London: Renewing harm reduction in the context of uncertainty. Current prac- tice patterns of primary care physicians in the management of patients with hepatitis C. Hepatitis B vaccination coverage levels among healthcare workers in the United States, 2002-2003. Adolescents’ knowledge, beliefs, and behaviors regarding hepatitis B: Insights and implications for programs targeting vaccine-preventable diseases. Hepatitis C disease among injection drug us- ers: Knowledge, perceived risk and willingness to receive treatment. Effectiveness of the hands-free tech- nique in reducing operating theatre injuries. Hepatitis B awareness, testing, and knowledge among Vietnamese American men and women. Hepatitis B knowledge and practices among Cambodian American women in Seattle, Washington. Hepatitis and Liver Cancer: A National Strategy for Prevention and Control of Hepatitis B and C http://www. Hepatitis B knowledge among Vietnamese immigrants: Implications for prevention of hepatocellular carcinoma. Hepatitis B knowledge and practices among Chinese immigrants to the United States. Hepatitis B knowledge and practices among Chinese American women in Seattle, Washington. Nonhospital health care-associated hepatitis B and C virus transmission: United States, 1998-2008. Seroepidemiology of hepa- titis B virus infection: Analysis of mass screening in Hawaii. Randomized controlled trial of a brief behavioural intervention for reducing hepatitis C virus risk practices among injecting drug users. Exploring risk perception and behaviour of homeless injecting drug users diagnosed with hepatitis C. Hepatitis B and liver cancer knowledge and preventive practices among Asian Americans in the San Francisco bay area, Califor- nia. San Francisco Hep B Free: A proactive approach to promoting hepatitis B immunization in conjunc- tion with screening and care. Hepatitis and Liver Cancer: A National Strategy for Prevention and Control of Hepatitis B and C http://www. Hepatitis and Liver Cancer: A National Strategy for Prevention and Control of Hepatitis B and C http://www. The frst Hpart of this chapter reviews current federal vaccination recommenda- tions and state vaccination requirements for hepatitis B. It also summarizes what is known about hepatitis B vaccination rates in specifc populations (for example, infants, children, and adults, including subgroups of at-risk adults, such as incarcerated people and occupationally exposed people). The committee identifed missed opportunities for hepatitis B vaccination and makes recommendations to increase the vaccination rate among vari- ous populations. The second part of this chapter summarizes current efforts to develop a hepatitis C vaccine and challenges that have been encountered. It is an anticancer vaccine: by preventing hepatitis B, it prevents hepatocellular carcinoma. Hepatitis and Liver Cancer: A National Strategy for Prevention and Control of Hepatitis B and C http://www. Of the three licensed combination vaccines, Twinrix® (GlaxoSmithKline Biologicals, Rixensart, Belgium) is used for vaccination of adults, and Comvax® (Merck & Co. The hepatitis B vaccine is administered in a three-dose series: two priming doses administered 1 month apart and a third dose administered 6 months after the second (Mast and Ward, 2008). The duration of immunity appears to be long-lasting, and booster doses of the vaccine are not routinely recommended (Mast and Ward, 2008). It is also used after liver transplantation for end-stage hepatitis B to prevent recurrence of the disease in the transplanted liver.

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It will not be necessary to change the code in everyone’s computer in the hospital buy super p-force oral jelly american express erectile dysfunction creams and gels, as is done today cheap super p-force oral jelly 160mg free shipping impotence newsletter, to upgrade or improve a com- puting application safe 160 mg super p-force oral jelly erectile dysfunction jackson ms. Responsibility for keeping the system operating smoothly and continuously is the vendor’s buy super p-force oral jelly overnight erectile dysfunction doctor in pakistan, not the hospital’s. The intelligence will be in the network the hospital (or physician or other user) taps into. This will be particularly helpful for smaller hospitals that could not afford advanced computer applications under the old model. They will pay for sophisticated computer applications, like the clin- ical navigational system described in Chapter 2, on a subscription basis depending on how much they use the services. Application service providers will also make it possible for large or small hospitals to share administrative support with smaller hos- Hospitals 61 pitals or physician groups on an as-needed basis. There has been a shared-services tradition in hospitals for the past 30 years; the Inter- net will dramatically expand the capacity to share services and help hospitals large and small reduce their clerical and administrative staffing to concentrate their scarce resources on the clinical services that patients see and use. Consumers will probably not notice any tangible difference be- tween health services supported from inside the hospital and those supported by remote computing, except that the systems will be faster and waste less of their time in registration, billing, and other consumer-facing functions. However, hospitals will save money us- ing these services that can be used to retain their nursing staffs and provide better customer service. It Won’t Happen Overnight The emergence of these outsourcing capabilities will not happen overnight. This phenomenon of outsourced hospital administrative and clinical services can be expected to emerge, not over a few months, but over the next ten years, driven by the successful execution of re- mote computing models. In a decade, business process outsourcing may be a $100 billion business in healthcare. The trend will also will be accelerated by the periodic cash flow and capital fund- ing crises that hospitals experience. As with any change in hospital operations, fierce cultural re- sistance to shared administrative and clinical services can be ex- pected from hospital department heads and the physicians they support. A bold attempt in the late 1990s by a large Catholic hospital chain, Catholic Healthcare West, to “virtualize” administrative and support services across their system was a colossal multi-hundred- million-dollar failure. This failure was due to poor execution and fierce resistance from local and regional hospital bureaucracies. Nevertheless, a more agile, responsive, and networked hospital system seems an inevitable, if painful, adaptation to an era of con- strained public and private healthcare payment. Steven Goldman of Lehigh University has written extensively on this networked man- agement model, which he has termed “agility. Many of the newer firms simply did not have the time to construct completely integrated manu- facturing and marketing functions. Competitive exigencies forced them to craft electronic networks of suppliers and distributors to bring their technologies to market. Many of the older firms that made this adaptation in automobile manufacture, steel fabrication, and so on, did so because they faced ruin from overseas competition and pressure from their customers for higher product quality and more responsive customer service. It is a troublesome reality that hospitalization exposes patients to risks that have nothing to do with their reason for being admitted in the first place. These risks include the risk of hospital-borne infections, adverse drug reactions, anesthesia problems, and other potential preventable threats to patient safety. Hospital executives have been uncertain of how to respond to reports of the prevalence Hospitals 63 of patient safety problems. Automating clinical processes is still very expensive, and hospital executives continue to question how signif- icant an economic return these technologies will generate. Only if board and management leadership are intolerant of the excuses for delivering a substandard product to the communities they serve. Chapter 8 discusses how to anticipate the problems of transforming clinical and management cultures and how hospital managers, boards, and medical staffs can approach this challenge with their eyes open. Trails Other English Speaking Countries in the Use of Electronic Medical Records and Electronic Prescribing. Despite the slings and arrows of man- aged care, physicians are also among the wealthiest professionals in the United States. Wealth and power, however, have not brought physicians the peace or sense of satisfaction one would have hoped. Published reports on physician practices suggest that significant numbers of physicians plan to retire in their 50s, well short of a full professional career. Sadly, given how important their work is, physicians function in an environment of barely contained chaos. Most physicians practice in two places: the hospital (whose troubled information systems were discussed in the previous chapter) and their offices. In the vast majority of cases, there is no functioning information link between these two sites. Moreover, physicians’ offices are awash in paper—patient rec- ords, prescriptions, medical journals, faxes, and telephone messages. Technically sophisticated in their personal and professional lives, 67 physicians have nonetheless lagged in adopting modern information technology to support their practices. The pressures of rising health costs, particularly on private employ- ers, encouraged an increased adoption of managed care. Narrowly construed, managed care involved establishing contractual relation- ships between physicians, hospitals, and other providers and health plans that limited the cost of care to predefined rates. However, more broadly, these contracts gave health insurers the power to review and modify physicians’ treatment plans to ensure that they were medically appropriate (with the goal of minimizing the cost). The advent of managed care contracts massively complicated the business operations of most medical practices. Because there are hundreds of health insurance plans with different cov- erage, review criteria, rates, and administrative procedures, health providers of all stripes found themselves bound like Gulliver by an emerging bureaucratic enterprise whose fundamental economic purpose was hostile to their own. The practical reality of these changes was that physicians could not count on being paid for medical care that cost more than a few hundred dollars without obtaining prior approval from a health plan. Physicians were forced to double or triple their office staffs, in some cases, to manage all these new transactions, which de- pended largely on telephone calls, fax transmittals, and written cor- respondence. The increasingly complex logistics of medical practice claimed an increasing percentage of the physicians’ workday, sub- tracting from time available for patients and family. No one likes having his or her professional judgment or moral commitment ques- tioned. It is not difficult to understand why the diminution of pro- fessional autonomy, incomes, and moral authority that physicians have experienced in the past decade would be unpleasant and stress- ful to them. But the increasing logistical complexity of physician practice has also taken a hidden toll on physicians. It has interfered with their intellectual development and ability to continue growing as professionals. They were the children who took things apart to see how they worked (and often succeeded in putting them back together).

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These may be important in the overall increase in medical knowledge best order super p-force oral jelly erectile dysfunction natural cure, 90 Essential Evidence-Based Medicine but not immediately important to an individual patient discount super p-force oral jelly 160mg amex erectile dysfunction weight loss. In fact cheap 160 mg super p-force oral jelly amex how does an erectile dysfunction pump work, these results buy 160 mg super p-force oral jelly with mastercard impotence qigong, called surrogate endpoints, may not translate into improved health at all. The researcher would look at all of the scores and correlate them with the date the test is taken. The researcher finds that there is a strong association between board scores and date, with the higher scores occurring among students taking the boards at earlier dates. The study would conclude that medical students should be taking the boards as early as possible in the cycle. What the researcher might be missing is that the timing of taking the exam and the score are both depen- dent on another factor, class rank. Final concerns There are a few more miscellaneous concerns for validity when evaluating out- come measurements. Were all of the important outcomes included and reported upon or were only certain main outcomes of the research project included? If cer- tain outcomes were measured to the exclusion of others, suspect foul play. A study may find a significant improvement in one outcome, for instance disease- free survival, while the outcome of importance for patients is overall survival, which shows no improvement. The problems associated with subgroup analy- sis and composite endpoints will be discussed in the chapter on Type I errors (Chapter 11). There is a definite publicationbias toward the publication of studies that show a positive result. Studies that show no effect or a negative result are more difficult to get published or may never be submitted for publication. Authors are aware of the decreased publication of negative studies, and as a result, it takes longer for negative studies to be written. The action of chance error causes distortion of the study results in a random way. Researchers can account for this problem with the appropriate use of statistical tests, which will be addressed in the next several chapters. Studies supported by or run by drug companies or other proprietary inter- ests are inherently biased. Since these companies want their products to do well in clinical trials, the methods used to bias these studies can be quite sub- tle. Drug-company sponsorship should be a red flag to look more carefully for sources of bias in the study. In general, all potential conflicts of interest should be clearly stated in any medical study article. Many journals now have mandatory Sources of bias 91 Table 8. Looking for sources of bias: a checklist Check the methods section for the following (1) The methods for making all the measurements were fully described with a clearly defined protocol for making these measurements. You will know more about this as you learn more background material about the subject. However, as the examples below illustrate, there are still some problems with this policy. In one case, Boots Pharmaceuticals, the maker of Synthroid, a brand of levothyroxine, a thyroid hormone commonly taken to replace low thyroid levels, sponsored a study of their thyroid hormone against generic thyroid replacement medication. That news was leaked to the Wall Street Journal, which published an account of the study. In the second case, a researcher at the Hospital for Sick Children in Toronto was the principal investigator in a study of a new drug to prevent the side effect of iron accumulation in children who needed to receive multiple trans- fusions. When the researcher attempted to make this information known to authorities at the uni- versity, the company threatened legal action and the researcher was removed 92 Essential Evidence-Based Medicine from the project. When other scientists at the university stood up to support the researcher, the researcher was fired. When the situation became public and the government stepped in, the researcher was rehired by the university, but in a lower position. The issues of conflict of interest in clinical research will be dis- cussed in more detail in Chapter 16. If readers think bias exists, one must be able to demonstrate how that bias could have affected the study results. Benjamin Disraeli, Earl of Beaconsfield (1804–1881) Learning objectives In this chapter you will learn: r evaluation of graphing techniques r measures of central tendency and dispersion r populations and samples r the normal distribution r use and abuse of percentages r simple and conditional probabilities r basic epidemiological definitions Clinical decisions ought to be based on valid scientific research from the medical literature. The competent interpreter of these studies must understand basic epidemiolog- ical and statistical concepts. Critical appraisal of the literature and good medical decision making require an understanding of the basic tools of probability. It is virtually impossible to describe the operations of a given biological system with a single, simple formula. Since we cannot mea- sure all the parameters of every biological system we are interested in, we make approximations and deduce how often they are true. Because of the innate vari- ation in biological organisms it is hard to tell real differences in a system from random variation or noise. Statistics seek to describe this randomness by telling us how much noise there is in the measurements we make of a system. By filter- ing out this noise, statistics allow us to approach a correct value of the underlying facts of interest. These include techniques for graphically displaying the results of a study and mathematical indices that summarize the data with a few key numbers. These key numbers are measures of central tendency such as the mean, median,andmode and measures of dispersion such as standard devia- tion, standard error of the mean, range, percentile,andquartile. In medicine, researchers usually study a small number of patients with a given disease, a sample. What researchers are actually interested in finding out is how the entire population of patients with that disease will respond. Researchers often compare two samples for different characteristics such as use of certain therapies or exposure to a risk factor to determine if these changes will be present in the population. Inferential statistics are used to determine whether or not any differences between the research samples are due to chance or if there is a true difference present. Also inferential statistics are used to determine if the data gathered can be generalized from the sample to a larger group of subjects or the entire population. Visual display of data The purpose of a graph is to visually display the data in a form that allows the observer to draw conclusions about the data. The reader is responsible for evaluating the accu- racy and truthfulness of graphic representations of the data.

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