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 did receive the exposure of interest, and one which did not, and following these cohorts forward for the outcome of interest.

Confidence interval (CI)

- the range within which we would expect the true value of a statistical measure to lie. The CI is usually accompanied by a percentage value which shows the level of confidence that the true value lies within this range. For example, for an NNT of 10 with a 95% CI of 5 to 15, we would have a 95% confidence that the true NNT value was between 5 and 15

Control event rate (CER)

– see treatment effects

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 uncertainiy

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 (see below) to all professions associated with health care, including purchasing and management.

Evidence-based medicine

- 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 means integrating individual clinical expertise and our patients' own values and expectations with the best available external clinical evidence from systematic research. See also Sackett DS et al : EBM : What it is and what it isn’t. BMJ: (1996) 312: 71-2

Experimental event rate (EER)

– see treatment effects

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 a target disorder

Intention-to-treat analysis

– a method of analysis for randomized trials in which all patients randomly assigned to one of the treatment are analyzed together, regardless of whether or not they completed or received that treatment

Likelihood ratios (LR)

–  the likelihood that a given test result would be expected in a patient with the target disorder compared with the likelihood that the same result would be expected in a patient without the target diorder ·         positive likelihood ratio (LR +) - a measure of how much a positive test pushes you towards making  the diagnosis. The greater the positive LR, the better the test is at diagnosing the target disorder. Likelihood ratios of 10 or more indicate a very useful test - when applied to pre-test probabilities of 33% or more, such tests will generate post-test probabilities of 83% or more. ·         negative likelihood ratio (LR +) - a measure of how much a negative test pushes you away from making  the diagnosis. The lower the negative LR, the better the test is at excluding the target disorder. Likelihood ratios of 0.1 or less indicate a very useful test -when applied to pre-test probabilities of 33% or less, such tests will generate post-test probabilities of 5% or less. Calculation of sensitivity/specificity/LR   Disease positive Disease negative Test positive a b Test negative c d   ·         Senstivity = a/(a + c) ·         LR+ = [senstivity/(1 – specificity)] = [a/(a + c)] ¸ [b/(b+d)] ·         Specificity = d/(b+d) ·         LR- = (1-sensitivity)/specificity = [c/(a +c)] ¸ [d/(b+d)] ·         pre-test probability = (a + c)/(a + b + c + d)

Number needed to follow (NNF)

- the number of patients that need to be followed to see one bad outcome: the lower the NNF, the more common the outcome. NNF = 1/(event rate)

Number extra needed to follow (NNF+)

- the number of patients with a certain risk factor (compared to without that risk factor) that need to be followed to see one extra bad outcome: the lower the NNF+, the worse the risk factor. Calculation of number extra needed to follow ·         using odds ratios  NNF+  = { PEER (OR - 1) + 1}/  {PEER (OR - 1) x (1 – PEER)} ·         using relative risks  NNF+ = 1 / { PEER (1 – RR)}

Number needed to harm (NNH)

- the number of patients that need to be treated to cause one bad outcome: the lower the NNH, the more harmful the treatment. See treatment effects.

Number needed to treat (NNT)

–- the number of patients that need to be treated to prevent one bad outcome: the lower the NNT, the better the treatment. See treatment effects.

Odds ratios  (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 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 the control subjects (without the target disorder). Calculations or OR/RR for trimethoprim-sulfamethoxazole prophylaxis in cirrhosis   adverse outcome occurs (infectious complication) adverse outcome does not occur (no infectious complication) totals exposed to treatment (experimental) 1 a 29 b 30 a+b not exposed to treatment (control) c   9 d   21 c+d   30 totals 10            a+c b+d          50 a+b+c+d   60   ·         CER = c/(c + d) = 30% ·         EER = a/(a + b) = 3.3% ·         control event odds  = c/d = 0.43 ·         experimental event odds = a/b = 0.034 ·         relative risk  = EER/CER = 0.11 ·         relative odds = odds ratio = (a/b)/(c/d) = ad/bc = 0.08

"Outcomes" study

- the observation of a defined population at a single point in time or time interval. Exposure and outcome are determined simultaneously.

Overview

– see systematic review

Patient expected event rate

– – see treatment effects

Post-test probability

- the proportion of patients with a particular test result who have the target disorder. Patients with suspected pulmonary embolism with a high probability ventilation-perfusion scan have a post-test probability of  82% i.e. most, but not all having a PE. Alternatively consider any patient with a high probability v/q scan as having a 82% chance of having a PE.

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). For example for patients referred to hospital with suspected pulmonary embolism, the pre-test probability is 30%; that is only a third actually have a PE confirmed on subsequent investigations. Alternatively consider any patient with a suspected PE as having a 30% chance of actually having one.

Randomised controlled trial (RCT)

- a group of patients is randomised into an experimental group (which receives the intervention under study) and a control group (which receives standard therapy or placebo). These groups are followed up for the variables/ outcomes of interest.

Relative risk (RR)

– the incidence (or risk) of an adverse outcome in patients with a prognostic factor relative to the risk in patients without that factor (equivalent to risk ratio)

Relative risk reduction (RRR)

- see treatment effects

Risk ratio (RR)

the ratio of risk in the treated group (EER) to the risk in the control group (CER) – used in randomized trials and cohort studies (equivalent to relative risk). RR = EER/CER

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.

Treatment effects

– the evidence-based medicine 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 haemoglobin A1c levels to 60% from 30%, and increased the proportion of patients with at least one episode of symptomatic hypoglycaemia to 47% to 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 calculation to describe these effects of treatment:

When the experimental treatment reduces the probability of a bad outcome (worsening diabetic retinopathy)

RRR (relative risk reduction) – , calculated as ½EER – CER½/CER, and accompanied by a 95% confidence interval (CI). In the case of worsening diabetic retinopathy, ½ EER – CER½/CER = ½13% - 38%½/ 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 ½EER – CER½, and accompanied by a 95% CI. In this case,½EER – CER½ = ½13%

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

Calculations for the occurrence of diabetic retinopathy in IDDMs

RBI (relative benefit increase) – the proportional increase in rates of good outcomes between experimental and control patients in a trial, calculated as ½EER – CER½/CER, and accompanied by a 95% confidence interval (CI). In the case of satisfactory haemoglobin A1c levels, ½ EER – CER½/CER = ½60% - 30%½/ 30% = 100%

ABI (absolute benefit  increase) – the absolute arithmetic difference in rates of good outcomes between experimental and control patients in a trial, calculated as½EER – CER½, and accompanied by a 95% CI. In this case,½EER – CER½ = ½60% – 30%½ = 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

RRI (relative risk increase) - the proportional increase in rates of bad outcomes between experimental and control patients in a trial, calculated as ½EER – CER½/CER, and accompanied by a 95% confidence interval (CI). In the case of hypoglycaemic episodes, ½ EER – CER½/CER = ½57% - 23%½/ 57% = 60%. (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 ½EER – CER½, and accompanied by a 95% CI. In this case,½EER – CER½ = ½57% – 23%½ = 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 lead to one additional patient being harmed, compared with patients who received the control treatment, calculated as 1/ARR, and accompanied by a 95% CI. In this case, 1/ARR = 1/34% = 3.