How the estimate a smple size ?

Hi everybody;

I am a PhD student, my thesis focuses on the study of reproductive health among teenagers girls .

For that, I'am going to work in public and private schools in two differents cities. I don't know, if i must choose the same effectif among public schools and privates school, knowing that in public school we have an important number of girls (i dont really have a good base in biostatistic :))

Also, i am going to work in two differents cities ( one is big and other is small) .As you know the sampling step is crucial in the research, so i ask for your help in two levels:
- what i can do to calculate exactlly my sample size (formula please) ? Can i consider my population as all number of inhabitants or just took into consideration the total number of girls in schools ?
- Must i choose a stratified smpling (age brackets) or a simply a random sampling ?

Sorry my english is not very prefect.

Thank You


Omega Contributor
You have to make decisions based on hypotheses on the minimally detectable effect size of interest, etc. Your familiarity with the content area has to drive the study aims and proposed analyses.