### A

- Absolute risk reduction (ARR)
- See treatment effects.
- Allocation concealment
- Occurs when the person who is enrolling a participant into a clinical trial is unaware whether the next participant to be enrolled will be allocated to the intervention or control group.

### C

- Case-control study
- A study which involves identifying patients who have the outcome of interest (cases) and patients without the same outcome (controls), and looking back to see if they had the exposure of interest.
- Case series
- A report on a series of patients with an outcome of interest. No control group is involved.
- Clinical Practice Guideline
- A systematically developed statement designed to assist clinician and patient decisions about appropriate health care for specific clinical circumstances.
- Cohort Study
- Involves identification of two groups (cohorts) of patients, one which received the exposure of interest, and one which did not, and following these cohorts forward for the outcome of interest.
- Confidence Interval (CI)
- Quantifies the uncertainty in measurement. It is usually reported as a 95% CI which is the range of values within which we can be 95% sure that the true value for the whole population lies. For example, for an NNT of 10 with a 95% CI of 5 to 15, we would have 95% confidence that the true NNT value lies between 5 and 15.
- Control Event Rate (CER)
- See treatment effects.
- 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 an intervention as well as the outcomes obtained. Outcomes are reported in a single unit of measurement.
- Cost-minimization analysis
- If health effects are known to be equal, only costs are analyzed and the least costly alternative is chosen.
- Cost-utility analysis
- Converts health effects into personal preferences (or utilities) and describes how much it costs for some additional quality gain (e.g. cost per additional quality-adjusted life-year, or QALY).
- Crossover study design
- The administration of two 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.

### D

- Decision analysis (or clinical 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.

### E

- 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.
- 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. Control event rate (CER) and experimental event rate (EER) are used to refer to this in control and experimental groups of patients, respectively. The patient expected event rate (PEER) refers to the rate of events we’d expect in a patient who received no treatment or conventional treatment. See treatment effects.
- Evidence-based health care
- Extends the application of the principles of evidence-based medicine to 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 evidence-based medicine requires the integration of individual clinical expertise with the best available external clinical evidence from systematic research and our patient’s unique values and circumstances.
- Experimental event rate (EER)
- The proportion of patients in the experimental treatment group who are observed to experience the outcome of interest. See treatment effects.

### H

- 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.

### I

- Incidence
- The proportion of new cases of the target disorder in the population at risk during a specified time interval.
- Inception cohort
- A group of patients who are assembled near the onset of the target disorder.
- Intention-to-treat analysis
- A method of analysis for randomized trials in which all patients randomly assigned to one of the treatments are analyzed together, regardless of whether or not they completed or received that treatment, in order to preserve randomization.

### L

- Likelihood ratio (LR)
- The likelihood that a given test result would be expected in a patient with the target disorder compared with the likelihood that this same result would be expected in a patient without the target disorder.

### M

- Meta-analysis
- A systematic review that uses quantitative methods to synthesize and summarize the results.

### N

- 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 physician 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
- Proportion of people with a negative test who are free of the target disorder. See likelihood ratio.
- Number needed to treat (NNT)
- The inverse of the absolute risk reduction and the number of patients that need to be treated to prevent one bad outcome. Calculated as the inverse of the absolute risk reduction (1/ARR). See treatment effects.

### O

- Odds
- A ratio of the number of people incurring an event to the number of people who don’t have an event.
- Odds ratio (OR)
- The ratio of the odds of having the target disorder in the experimental group relative to the odds in favour of having the target disorder in the control group (in cohort studies or systematic reviews) or the odds in favour of being exposed in subjects with the target disorder divided by the odds in favour of being exposed in control subjects (without the target disorder).
- Overview
- See systematic review.

### P

- Patient expected event rate
- See treatment effects.
- Positive predictive value
- Proportion of people with a positive test who have the target disorder. See likelihood ratio.
- 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).
- Post-test probability
- The proportion of patients with that particular test result who have the target disorder (post-test odds/[1 + post-test odds]).
- Pre-test odds
- The odds that the patient has the target disorder before the test is carried out (pre-test probability/ [1 – pre-test probability]).
- Pre-test probability (prevalence)
- The proportion of people with the target disorder in the population at risk at a specific time (point prevalence) or time interval (period prevalence). See likelihood ratio.

### R

- Randomization (or 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 control clinical trial (RCT)
- Participants are randomly allocated into an experimental group or a control group and followed over time for the variables/outcomes of interest.
- Relative risk reduction (RRR)
- See treatment effects.
- 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. Also called relative risk.

### S

- Sensitivity
- Proportion of people with the target disorder who have a positive test result. It is used to assist in assessing and selecting a diagnostic test/sign/symptom. See likelihood ratio.
- SnNout
- When a sign/test/symptom has a high Sensitivity, a Negative result can help rule out the diagnosis. For example, the sensitivity of a history of ankle swelling for diagnosing ascites is 93%; therefore if a person does not have a history of ankle swelling, it is highly unlikely that the person has ascites.
- Specificity
- Proportion of people without the target disorder who have a negative test. It is used to assist in assessing and selecting a diagnostic test/sign/symptom. See likelihood ratio.
- 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.
- Systematic review
- A summary of the medical literature that uses explicit methods to perform a comprehensive literature search and critical appraisal of individual studies and that uses appropriate statistical techniques to combine these valid studies.

### T

- Treatment effects
- The evidence-based journals (Evidence Based Medicine and ACP Journal Club) have achieved consensus on some terms they use to describe both the good and bad effects of therapy. We will bring them to life with a synthesis of three randomized trials in diabetes which individually showed that several years of intensive insulin therapy reduced the proportion of patients with worsening retinopathy to 13% from 38%, raised the proportion of patients with satisfactory hemoglobin A1c levels to 60% from about 30%, and increased the proportion of patients with at least one episode of symptomatic hypoglycemia to 57% from 23%. Note that in each case the first number constitutes the “experimental event rate” (EER) and the second number the “control event rate” (CER). We will use the following terms and calculations to describe these effects of treatment: When the experimental treatment reduces the probability of a bad outcome (worsening diabetic retinopathy)
- RRR (relative risk reduction)
- The proportional reduction in rates of bad outcomes between experimental and control participants in a trial, calculated as |EER – CER|/CER, and accompanied by a 95% confidence interval (CI). In the case of worsening diabetic retinopathy, |EER – CER|/CER = |13% – 38%|/38% = 66%.
- ARR (absolute risk reduction)
- The absolute arithmetic difference in rates of bad outcomes between experimental and control participants in a trial, calculated as |EER – CER|, and accompanied by a 95% CI. In this case, |EER – CER| = |13% – 38%| = 25%. (This is sometimes called the risk difference)
- NNT (number needed to treat)
- The number of patients who need to be treated to achieve one additional favorable outcome, calculated as 1/ARR and accompanied by a 95% CI. In this case, 1/ARR = 1/25% = 4.

When the experimental treatment increases the probability of a good outcome (satisfactory hemoglobin A1c levels):

- RBI (relative benefit increase)
- The proportional increase in rates of good outcomes between experimental and control patients in a trial, calculated as |EER – CER|/CER, and accompanied by a 95% confidence interval (CI). In the case of satisfactory hemoglobin A1c levels, |EER – CER|/CER = |60% – 30%|/30% = 100%.
- ABI (absolute benefit increase)
- The absolute arithmetic difference in rates of good outcomes between experimental and control patients in a trial, calculated as |EER – CER|, and accompanied by a 95% confidence interval (CI). In the case of satisfactory hemoglobin A1c levels, |EER – CER| = |60% – 30%| =30%
- NNT (number needed to treat)
- The number of patients who need to be treated to achieve one additional good outcome, calculated as 1/ARR and accompanied by a 95% CI. In this case, 1/ARR = 1/30% = 3.

When the experimental treatment increase the probability of a bad outcome (episodes of hypoglycemia):

- RRI (relative risk increase)
- The proportional increase in rates of bad outcomes between experimental and control patients in a trial, calculated as |EER – CER|/CER, and accompanied by a 95% confidence interval (CI). In the case of hypoglycemic episodes, |EER – CER|/CER = |57% – 23%|/23% = 148%. (RRI is also used in assessing the impact of “risk factors” for disease.)
- ARI (absolute risk increase)
- The absolute arithmetic difference in rates of bad outcomes between experimental and control patients in a trial, calculated as |EER – CER|, and accompanied by a 95% confidence interval (CI). In the case of hypoglycemic episodes, |EER – CER| = |57% – 23%| = 34%. (ARI is also used in assessing the impact of “risk factors” for disease.)
- NNH (number needed to harm)
- The number of patients who, if they received the experimental treatment, would result in one additional patient being harmed, compared with patients who received the control treatment, calculated as 1/ARI and accompanied by a 95% CI. In this case, 1/ARI = 1/34% = 3.