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Glossary Absolute risk reduction (ARR) Measure of the efficacy of a treatment in terms of the absolute number of people saved (or lost, in the case of an absolute risk increase). For instance, if a treatment reduces the number of people who die of a disease from 6 to 4 in 1,000, then the absolute risk reduction is 2 in 1,000, or 0.2 percentage points, while the relative risk reduction is 33% (see below). Absolute risk reductions are transparent, while relative risk reductions tend to mislead people. Basicnumeracy Abilitytounderstandsimplepercentages,absolutenumbers,andchance. Bias A systematic error in the design, conduct, or the analysis of a study. Citizen A native or inhabitant of a particular place; used synonymously with “person ” or “member of society.” Usage is not meant to exclude on the basis of nationality or citizenship. Conditional probability The probability that an event A occurs given event B, usually written as p(A|B). An example of a conditional probability is the probability of a positive screening mammogram given breast cancer, which is around .90. The probability p(A), for instance, is not a conditional probability . Conditional probabilities are notoriously misunderstood, in two different ways. One is to confuse the probability of A given B with the probability of A and B; the other is to confuse the probability of A given B with the probability of B given A. This confusion can be reduced by replacing conditional probabilities with natural frequencies. Decision support interventions or decision aids Tools designed to help people make informed health care decisions. They provide information about the options and outcomes and assist in the clarification of personal values. Decision aids are designed to complement, rather than replace, counseling from a health practitioner. Drug bulletins Periodicals that report on issues related to drug therapy. They are produced independently of the pharmaceutical industry and aim to provide balance to the large number of journals that depend on advertising by manufacturers . False positive rate The proportion of positive tests among individuals who do not have a disease (or condition). It is typically expressed as a conditional probability or a percentage. For instance, mammography screening has a false positive rate of 5–10% depending on age; that is, 5–10% of women without breast cancer receive nevertheless a positive test result. The false positive rate and the specificity (power) of a test add up to 100%. The rates of the two possible errors, false positive and false negatives (misses), are dependent: decreasing the false positive rate of a test increases the false negative rate, and vice versa. Health literacy The degree to which individuals can obtain, process, and understand basic knowledge about diseases, causes, prevention, diagnostics, treatment, and services they need to make appropriate health decisions. Health system literacy The degree to which individuals understand how the health system works, including knowledge about the incentives within it, such as the widespread practice of defensive medicine as a reaction to the threat of litigation. 344 Glossary Illusion of certainty Belief that an event is absolutely certain, even though it may not be. For instance, people tend to believe that the results of modern technologies (e.g., DNA fingerprinting) or medical tests (e.g., HIV testing ) are certain, that is, is error-free. The illusion can have benefits, such as reassurance, but it also has costs, such as suicide after a false positive HIV test. Its origin is not simply cognitive, but often social. For instance, with respect to political or religious values, the illusion of certainty may be a requirement for being accepted by a social group, fostering social control. Meta-analysis Astatistical technique used to combine the results from several studies that address a shared research hypothesis. Mismatched framing Practice of reporting benefits and harms of treatments differently : benefits are exaggerated, whereas harms are downplayed. This can be done in various ways, one of which is to frame the benefit as a relative risk reduction (big numbers) and the harms as an absolute risk increase (small numbers). Natural frequencies A transparent form of risk communication. While conditional probabilities (such as sensitivity and false positive rate) make it difficult for most physicians to infer the positive predictive value of a test, natural frequencies make it easy. For instance, consider a disease with a prevalence of 4%, a test with a sensitivity of 80%, and a false positive rate of 5%. What is the probability that a patient has the...

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