i am planning an observational cohort research to see the relation of smoking with post surgery infection. independent variable is dichotomous with infection present or absent. i want to divide exposure into non-smokers, causal smokers and chain smoker. if all patients undergo same surgical procedure and i follow them for 10 days till suture removal. and then assess how many patients were infected in non-smokers, causal smokers and chain smokers. does it fulfill criteria of cohort study?

questions:

1. i want to calculate sample size(using openepi). i could not find a similar cohort study but observational study with incidence of infection in smokers and non smokers.can i use their percentages to derive sample size? or use 50% difference between exposed and nonexposed groups?

2. is it okay if i stick to the sample size and divide patients according to their smoking habit into 3 groups with no regard to ratio of exposed or unexposed group?

3. almost all patient return for stitch removal and i can assess them. do i still need to add 10% for loss to follow up?

(above study about smoking and infection is just an example my study pattern is same with different variables)

would appreciate your help.

regard,