Khalid Ahsan  ( Department of Anaesthesia, Aga Khan University, Karachi, Pakistan )

A cohort study can be done with single (exposure) group only. Although in this situation, the relative parameters as "relative risk" or "risk difference" cannot be calculated. In a single group cohort the absolute risk of the exposed patients can be measured.
We followed all patients who underwent cardiac surgeries during the study period. In this case no standard or hypothetical method of sample size calculation was found necessary.
Our intention was not to see the time period to event/outcome. The analysis of this data can also be done through survival analysis.
Each patient was enrolled after elective cardiac surgery and followed for 9th postoperative day. It was an individual exposure (cardiac surgery), so each patient was followed individually.