Evidence Based Medicine: when best evidence from research meets clinical information and patient values, optimal decisions are possible.
Absolute risk difference: The difference in the risk for disease or death between an exposed population and an unexposed population. (Harm/Etiology)
Adjustment: A summarizing procedure in which the effects of differences in composition of the populations being compared have been minimized by statistical methods. See Confounding variable (Harm/Etiology)
ARR : see
Absolute risk reduction
Blind assessment: The evaluation of an outcome is
made without the evaluator knowing which results are from the test under
study and which are from the control or gold standard.
Blind(ed) study (masked study): A study in which observer(s) and/or subjects are kept ignorant of the group to which the subjects are assigned, as in an experimental study, or of the population from which the subjects come, as in a nonexperimental or observational study. Where both observer and subjects are kept ignorant, the study is termed a double-blind study. If the statistical analysis is also done in ignorance of the group to which subjects belong, the study is sometimes described as triple blind. The purpose of "blinding" is to eliminate sources of bias. (Diagnosis, Harm/Etiology, Therapy)
Case-control study: Retrospective comparison of exposures of persons with disease (cases) with those of persons without the disease (controls) (Harm/Etiology). See Retrospective study.
Case-series: Report of a number of cases of disease. (Harm/Etiology)
Causality: The relating of causes to the effects they produce. Most of epidemiology concerns causality and several types of causes can be distinguished. It must be emphasized, however, that epidemiological evidence by itself is insufficient to establish causality, although it can provide powerful circumstantial evidence. (Harm/Etiology)
Control Event Rate
CI: see see
Clinical outcome: Measures patient health or well being. Ideally it should be credible, comprehensive, sensitive to change, accurate, sensible, and biologically sensible. Compare with Surrogate outcome. (Diagnosis, Harm/Etiology, Prognosis, Therapy)
Guideline: A systematically developed statement designed to assist
clinician and patient decisions about appropriate health care for specific
Co-interventions: Interventions other than the treatment under study that may have been applied differently to the study and control groups. Co-intervention is a serious problem when double-blinding is absent or when the use of very effective non-study treatments is permitted. (Therapy)
study: A study that begins with the gathering of two matched groups
(the cohorts), one which has been exposed to a prognostic factor,risk
factor or intervention and one which has not. The groups are then followed
forward in time (prospective) to measure the
development of different outcomes. In a retrospective
cohort study, cohorts are identified at a point of time in
the past and information is collected on their subsequent outcomes.
(Diagnosis, Harm/Etiology, Prognosis, Therapy)
An inception cohort is a group identified at the onset of a disorder or a first exposure to a potential risk factor, and followed forward in time. (Harm/Etiology, Prognosis)
Comparison group: Any group to which the index group is compared. Usually synonymous with control group. (Harm/Etiology, Therapy)
(CI): "The CI gives a measure of the precision (or uncertainty) of
study results for making inferences about the population of all such patients".
(Strauss, 2005 p. 263) The 95% CI is the range of values within which we can be
95% sure that the true value lies for the whole population of patients from
whom the study patients were selected. Wide confidence intervals indicate less
precise estimates of effect. CI is affected by sample size and by variability
among subjects. The larger the trial's sample size is, the larger the number of
outcome events and the greater the confidence that the true relative risk
reduction is close to the value stated: the confidence intervals narrow and
"precision" is increased.
variable (confounder): A characteristic that may be
distributed differently between the study and control groups and that can
effect the outcome being assessed. Confounding may be due to chance or
bias. See Adjustment, Selection bias
Cost-benefit analysis: Assesses whether the cost of an intervention is worth the benefit by measuring both in the same units; monetary units are usually used.
Cost-effectiveness analysis: Measures the net cost of providing a service as well as the outcomes obtained. Outcomes are reported in a single unit of measurement.
Crossover study design: The administration of 2 or more experimental therapies one after the other in a specified or random order to the same group of patients.
Cross-sectional study: The observation of a defined population at a single point in time or time interval. Exposure and outcome are determined simultaneously.
Decision analysis: The application of explicit, quantitative methods that quantify prognoses, treatment effects, and patient values in order to analyze a decision under conditions of uncertainty.
Dose-response relationship: A relationship in which change in amount, intensity, or duration of exposure is associated with a change--either an increase or decrease--in frequency or intensity of a specified outcome. (Harm/Etiology)
Determinant: Any definable factor that effects a change in a health condition or other characteristic. (Harm/Etiology)
Ecological survey: A survey based on aggregate data for some population as it exists at some point or points in time; to investigate the relationship of an exposure to a known or presumed risk factor for a specified outcome.
EER: see Experimental Event Rate
Effectiveness: A measure of the benefit resulting from an intervention administered under usual conditions of clinical care for a particular group of patients. See Intention to treat. (Therapy)
Etiology: The study of the cause or origin of a disease.
Event rate: The proportion of patients in a group in whom the event is observed. Thus if out of 100 patients, the event is observed in 27, the event rate is 0.27.
Evidence based health care: The conscientious, explicit and judicious use of current best evidence in making decisions about the care of individual patients. Involves all professions associated with health care including purchasing and management.
Evidence-based medicine (EBM): The conscientious, explicit and judicious use of current best evidence in making decisions about the care of individual patients. The practice of EBM means integrating individual clinical expertise with the best available external clinical evidence from systematic research.
Exclusion criteria: Stated conditions which preclude entrance of candidates into an investigation even if they meet the inclusion criteria. (Diagnosis, Harm/Etiology, Prognosis, Therapy)
Experimental event rate (EER): The
percentage of intervention/exposed group who experience outcome in
Follow-up: Observation over a period of time of an individual, group, or initially defined population whose relevant characteristics have been assessed in order to observe changes in health status or health-related variables. (Harm/Etiology, Prognosis, Therapy)
Gold standard (also Reference standard): Ideally, the criterion used to unequivocally define the presence of a condition; or practically, the method, procedure or measurement that is widely accepted as being the best available to detect the presence of a condition. (Diagnosis)
Heterogeneity: This occurs when there is more variation between the study results (in a systematic review) than would be expected to occur by chance alone.
Inception cohort: A group of patients who are assembled near the onset of the target disorder.
Incidence: The rate of new cases of illness commencing during a specified time period in a given population See also Prevalence. (Harm/Etiology)
Index test: The test whose diagnostic accuracy is being measured against the reference or gold standard. (Diagnosis)
Intention to treat analysis: Individual outcomes in a clinical trial are analyzed according to the group to which they have been randomized, regardless of whether they dropped out, fully complied with the intervention or crossed over to the other treatment. By simulating practical experience intention to treat analysis provides a better measure of effectiveness (as opposed to efficacy). (Therapy)
Lead-time bias: : Overestimation of survival because of earlier diagnosisâtime of death does not change, just time of diagnosis. (Harm/Etiology, Prognosis)
ratio: The likelihood ratio for a test result compares the likelihood
of that result in patients with disease to the likelihood of that result in
patients without disease.
Likelihood ratio of a negative test: Ratio of the probability of a false negative result if the disease is present to the probability of a true negative result if the disease is absent.
Meta-analysis: Statistical synthesis of the results from several studies that address the same question.
N-of-1 trials: In such trials, the patient undergoes pairs of treatment periods organized so that one period involves the use of the experimental treatment and the other involves the use of an alternate or placebo therapy. The patient and physicians are blinded, if possible, and outcomes are monitored. Treatment periods are replicated until the clinician and patient are convinced that the treatments are definitely different or definitely not different.
Negative predictive value: The proportion of people who receive a negative test result who are truly free of the target disorder.
NNT: see Number Needed to Treat
Number needed to harm (NNH): The number of patients for whom there is one additional patient who experiences a harmful outcome.
Observational study (non-experiemental study): Changes or differences in one characteristic (e.g. whether or not people received a specific treatment or intervention) are studied in relation to changes or differences in other(s) (e.g. whether or not they died), without the intervention of the investigator. Examples are case control and cohort studies.
Odds: A ratio between two probabilitiesâthe probability of an event to a non-event. (Etiology/Harm, Therapy)
OR: see Odds Ratio
P value: The possibility that any particular outcome would have occurred by chance. Statistical significance is usually p<0.05. Considered to be inferior to confidence intervals in determining significance of studies.
Patient Expected Event rate (PEER): This refers to the rate of events expected in a patient who received conventional therapy or no treatment.
PEER: see Patient Expected Event Rate
Positive predictive value: Proportion of people with a positive test who have the target disorder.
Post-test odds: The odds that the patient has the target disorder after the test is carried out (calculated as the pre-test odds x likelihood ratio).
Power:The ability of a study to demonstrate an
association or causal relationship between two variables, given that an
association exists. For example, 80% power in a clinical trial means that the
study has a 80% chance of showing a statistically significant treatment effect
if there really was an important difference between outcomes. If the
statistical power of a study is low, the study results will be questionable
(the study might have been too small to detect any differences).
By convention, 80% is an acceptable level of power. (Bandolier; April 1, 2008. http://www.jr2.ox.ac.uk/bandolier/booth/glossary/statpow.html )
Precision (statistical precision): The range in which
the best estimates of a true value approximate the true value.
(Diagnosis, Harm/Etiology, Prognosis, Therapy)
See Confidence interval.
Predictive value: In screening and
diagnostic tests, the probability that a person with a positive test is a true
positive (i.e., does have the target disease), or that a person with a negative
test truly does not have the disease. The predictive value of a screening test
is determined by the sensitivity and
specificity of the test, and by the prevalence of
the condition for which the test is used.
Primary research: Individual studies such as randomized controlled trials, cohort studies, case-control studies, cross-sectional studies, etc.
Prognostic factor: A factor or indicator (such as age or gender) related to an individualâs probability of developing a disease or other outcome. Compare with risk factors. Neither prognostic nor risk factorsnecessarily imply a cause and effect relationship. (Prognosis)
Prospective study: Study design where one or more groups (cohorts) of individuals who have not yet experienced the outcome event in question are followed forward in time and monitored for the number of such events which occur (Diagnosis, Harm/Etiology, Prognosis, Therapy)
Randomization (random allocation): Method analogous to tossing a coin to assign patients to treatment groups (the experimental treatment is assigned if the coin lands 'heads' and a conventional 'control' or placebo treatment is given if the coin lands 'tails').
Randomized controlled trial: An experimental comparison study in which participants are allocated via a randomization mechanism to either an intervention/treatment group or a control /placebo group, then followed over time and assessed for the outcomes of interest. Participants have an equal chance of being allocated to either group. (Therapy)
RCT: see Randomized control clinical trial
Reference standard: See Gold standard.
Referral bias: The sequence of referrals that may lead patients from primary to tertiary centers raises the proportion of more severe or unusual cases, thus increasing the likelihood of adverse or unfavorable outcomes. Physicians and medical centers may attract individuals with specific disorders or exposures. (Prognosis)
Relative risk (RR): The
ratio of the probability of developing, in a specified period of time, an
outcome among those receiving the treatment of interest or exposed to a risk
factor, compared with the probability of developing the outcome if the risk
factor or intervention is not present. (Therapy, Harm/Etiology)
study: Study design in which cases where individuals who had an
outcome event in question are collected and analyzed after the outcomes have
See also Case-control study.
Risk factor: Patient characteristics or factors associated with an increased probability of developing a condition or disease in the first place. Compare with prognostic factors. Neither risk nor prognostic factors necessarily imply a cause and effect relationship. (Harm/Etiology)
Risk ratio: The ratio of risk in the treated group (EER) to the risk in the control group (CER). This is used in randomized trials and cohort studies and is calculated as EER/CER.
RR: see Relative Risk
RRR: see Relative Risk Reduction
Sample size: Is the size of the sample. Larger samples usually mean more precise results. Sample size usually depends on the purpose of the study, the population size from which the sample the sample will be pulled, as well as the level of precision and the level of confidence or risk that is acceptable, and the degree of variability in the attributes being measured.
Selection bias: A bias in assignment or a confounding variable that arises from study design rather than by chance. These can occur when the study and control groups are chosen so that they differ from each other by one or more factors that may affect the outcome of the study. (Harm/Etiology, Therapy)
Sensitivity (of a
diagnostic test): The proportion of truly diseased persons, as
measured by the gold standard, who are identified
as diseased by the test under study. (Diagnosis)
SnNout: When a sign/test/symptom has a high Sensitivity, a Negative result rules out the diagnosis. For example, the sensitivity of a history of ankle swelling for diagnosing ascites is 93%; therefore is a person does not have a history of ankle swelling, it is highly unlikely that the person has ascites.
Specificity (of a
diagnostic test): The proportion of truly non-diseased persons, as
measured by the gold standard, who are so identified by the
diagnostic test under study. (Diagnosis)
SpPin: What a sign/test/symptom has a high Specificity, a Positive result rules in the diagnosis. For example, the specificity of a fluid wave for diagnosing ascites is 92%; therefore if a person does have a fluid wave, it rules in the diagnosis of ascites.
Stratification: Division into groups. Stratification may also refer to a process to control for differences in confounding variables, by making separate estimates for groups of individuals who have the same values for the confounding variable. (Therapy)
Strength of inference: The likelihood that an observed difference between groups within a study represents a real difference rather than mere chance or the influence of confounding factors, based on both p values and confidence intervals. Strength of inference is weakened by various forms of bias and by small sample sizes. (Harm/Etiology, Therapy)
Surrogate outcome/endpoint: Intended to capture the treatment effect of an important clinical endpoint but does not directly measure the clinical benefit of the intervention, substitutes something we can measure for something we want to measure. Compare with clinical outcome. (Diagnosis, Harm/Etiology, Prognosis, Therapy)
Survival curve: A graph of the number of events occurring over time or the chance of being free of these events over time. The events must be discrete and the time at which they occur must be precisely known. In most clinical situations, the chance of an outcome changes with time. In most survival curves the earlier follow-up periods usually include results from more patients than the later periods and are therefore more precise. (Prognosis)
Systematic review: A summary of the medical literature that uses explicit methods to perform a thorough literature search and critical appraisal of individual studies and that uses appropriate statistical techniques to combine these valid studies.
Test/treatment thresholds: The probability of disease above which we treat for the disease and below which we do not treat. The treatment threshold is determined by the costs and benefits of the treatment.
Values can be assigned to these thresholds from data on the reliability and potential risks of the diagnostic test and the benefits and risks of the diagnostic test and the benefits and risks of a specific treatment. Treatment should be withheld if the probability of disease is smaller than the testing threshold, and treatment should be given without further testing if the probability of the disease is greater than the test-treatment threshold. The test should be performed (with treatment depending on the test outcome) only if the probability of disease is between the two thresholds. The method exposes important principles of decision making and helps the clinician develop a rational, quantitative approach to the use of diagnostic tests.
The probability at which one should be indifferent between testing and treating.
Validity: The degree to which the results of a study are likely to be true, believable and free of bias. This is entirely independent of the precision of the results and does not predict the results to your patients. (Diagnosis, Harm, Prognosis, Therapy)
The internal validity of a study refers to the integrity of the experimental design.
The external validity of a study refers to the appropriateness by which its results can be applied to non-study patients or populations.
Verification bias (work-up bias): Occurs when patients with negative test results are not evaluated with the gold standard test. See also Bias.
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