Editing Epidemiology (section)

=== Cohort studies ===
[[Cohort studies]] select subjects based on their exposure status. The study subjects should be at risk of the outcome under investigation at the beginning of the cohort study; this usually means that they should be disease free when the cohort study starts. The cohort is followed through time to assess their later outcome status. An example of a cohort study would be the investigation of a cohort of smokers and non-smokers over time to estimate the incidence of lung cancer. The same 2×2 table is constructed as with the case control study. However, the point estimate generated is the [[relative risk]] (RR), which is the probability of disease for a person in the exposed group, ''P''<sub>e</sub>&nbsp;=&nbsp;''A''&nbsp;/&nbsp;(''A''&nbsp;+&nbsp;''B'') over the probability of disease for a person in the unexposed group, ''P''<sub>''u''</sub>&nbsp;=&nbsp;''C''&nbsp;/&nbsp;(''C''&nbsp;+&nbsp;''D''), i.e. ''RR''&nbsp;=&nbsp;''P''<sub>e</sub>&nbsp;/&nbsp;''P''<sub>u</sub>.

{| class="wikitable"
|-
! .....
! Case
! Non-case
! Total
|-
| Exposed
| ''A''
| ''B''
| (''A''&nbsp;+&nbsp;''B'')
|-
| Unexposed
| ''C''
| ''D''
| (''C''&nbsp;+&nbsp;''D'')
|}

As with the OR, a RR greater than 1 shows association, where the conclusion can be read "those with the exposure were more likely to develop the disease."

Prospective studies have many benefits over case control studies. The RR is a more powerful effect measure than the OR, as the OR is just an estimation of the RR, since true incidence cannot be calculated in a case control study where subjects are selected based on disease status. Temporality can be established in a prospective study, and confounders are more easily controlled for. However, they are more costly, and there is a greater chance of losing subjects to follow-up based on the long time period over which the cohort is followed.

Cohort studies also are limited by the same equation for number of cases as for cohort studies, but, if the base incidence rate in the study population is very low, the number of cases required is reduced by&nbsp;{{frac|1|2}}.