no. a cohort study can be done can be done for all exposures.

a simple power /sample size calculator is given at

https://epitools.ausvet.com.au/cohortss

Hello dr Ahmed

The answer is no. As there are 2 types of cohort.

Retrospective cohort can be done in outbreak investigation or epidemics or even pandemics as in COVID-19.

Retrospective cohort can be done in industrial facilities by archived records.

But prospective cohort is higher evidence based than retrospective cohort.

Regarding calculating sample size , there are many software , I recommend

Openepi online software

https://www.openepi.com/SampleSize/SSCohort.htm

And G.power software based on needed statistical data like effect size mainly.

The UK has a number of birth cohorts 1946, 1958 1970 , Millenium Cohort who were selected because they were born in the same year and then followed - there were no specific exposure of interest and a very wide range of data was collected on them

https://en.wikipedia.org/wiki/British_birth_cohort_studies

cohort studies can be done with rare exposures but "not only"

specially for rare exposure with high incidence of disease among the exposed.

epi-info is one of the software used for sample size calculation where it needs the ratio between exposed and unexposed, % outcome in unexposed group and % outcome in exposed group from any previous study done on the same topic.

Thanks All for yor answer what if, if there is not study done previously? i mean for cohort study sample size calculation?

You have to make an estimate about what level of RR you wish to detect.

usually based on seriousness of outcome (medically , socially and politically)

As far as I know, in Cohort studies, we should be aware that the outcome is not rare. I don't think so there is any rule for exposure as assigning in exposure and non-exposure groups is done by the researcher.