Study characteristics

Advantage

Disadvantage

Cost

 

Very high

Time

 

Long

Common condition

Good chance of getting cases

 

Rare condition

 

Need too big a cohort to detect even a few cases

Easily defined and measurable exposure

Yes.  Vaguely defined exposures are not good especially if standard definitions change during the study

 

Easily defined and measurable case condition

Yes.  Vaguely defined conditions are not good especially if standard definitions change during the study

 

Subject compliance

 

Due to length of study compliance with protocols and drop rates are problems

Incidence rate

Directly measured

 

Incidence rate ratio

Directly measured

 

Sample size

 

To achieve statistical significance for modest level of effect and to adjust for drop outs, a large sample size is needed