Study Designs

n How Research Is Classified

n Terminology

n Important epidemiologic

concepts

Descriptive Statistics

n Measures of central

tendency

n Measures of spread

n Measures of frequency of

events

n Measures of Association

n Terms used to describe the

quality of measurements

n Measures of diagnostic test

accuracy

n Expressions used when

making inferences about

data

n Multivariable Regression

Methods

References

relative risk in case-control studies. For example, a case-control

study was done to evaluate the relationship between artiﬁcial

sweeteners and bladder cancer. The odds of artiﬁcial sweetener

use in the cases and controls were used to calculate an odds

ratio and determine whether sweeteners were associated with

bladder cancer. Under the assumption that the disease under

consideration is rare (e.g. bladder cancer), the odds ratio gives a

stable, unbiased estimate of the relative risk (Figure 1). The odds

ratio from a case-control study nested within a deﬁned cohort

also approximates the relative risk even when the rare disease

assumption is not held.

If the disease is rare, A<<B and C<<D. So, A/(A + B) is approximated

by A/B and C/(C + D) approximated by C/D. In this situation, the

relative risk equals (A/B)/(C/D) which, rearranged, equals the odds

ratio A×D/B×C

Absolute risk The relative risk and odds ratio provide a measure

of risk compared with a standard. However, it is sometimes desir-

able to know the absolute risk. For example, a 40% increase in

risk of heart disease because of a particular exposure does not

provide insight into the likelihood that exposure in an individual

patient will lead to heart disease.

The Attributable risk or Risk difference is a measure of abso-

lute risk. It represents the excess risk of disease in those exposed

taking into account the background rate of disease. The attribut-

able risk is deﬁned as the difference between the incidence rates

in the exposed and non-exposed groups.

A related term, the Population Attributable Risk is used to de-

scribe the excess rate of disease in the total study population of

exposed and non-exposed individuals that is attributable to the

exposure. This measure is calculated by multiplying the Attributable

risk by the proportion of exposed individuals in the population.

Number needed to treat (NNT) The number of patients who

would need to be treated to prevent one adverse outcome is

often used to present the results of randomized trials. NNT is the

reciprocal of the absolute risk reduction (the absolute adverse

event rate for placebo minus the absolute adverse event rate for

treated patients). This approach can be used in studies of vari-

ous interventions including both treatment and prevention. The

estimate for NNT is subject to considerable error and is generally

presented with 95% conﬁdence intervals so that it can be prop-

erly interpreted.

Terms Used To Describe The Quality Of Measurements

Reliability The concept of reliability or reproducibility is related

to the amount of error in any measurement (e.g. blood pressure