The estimated sample size was 280 according to the population (registered pharmacist). Since I am going to use two diffrent prescriptions to evaluate their practice, will the sample be 280×2 or 280 ÷2 ?? Or is it calculate by another way?
It depends on what you are measuring in the practice - eg if it is % of a certain type of prescription fillable by generics (ie a prevalence to be measured with a desired precision) and the sample size is estimated as 280 for one type then another type of prescription may have a different population prevalence and a different sample size would have to be estimated.
On the other hand you were assessing the mean time to fill prescriptions for two common drugs( both equally available) with a certain precision,then the same sample size could be used for both types of prescription.
It sounds like you are trying to estimate the prevalence in the population, in which case the single overall sample of 280 can be used for each.
Alternatively, if you have hypotheses that you are going to evaluate, then you should look into an alternative method of estimating sample size using G*power to assess the "power" of your test.
It depends on design of study: what measurement you corrected.
It depends on your working hypothesis. The sample size of community pharmacists would remain the same and the prescription population would change with the two prescriptions being reviewed. An analysis of the contrast and comparison of pharmacy fill practice and rationale would
vary by prescription.
In my opinion, you do not need to double the sample size because both prescriptions measure the same variables, "the ability to detect the drug-drug interactions and to provide the proper counselling." But when you want to describe the type of the identified drug-drug interaction, which represented by one question in your data collection tool, you must mention it with regard to the total number of surveyed pharmacies for each prescription.
But the question here, what is the point of using two prescriptions. Why don't you use only one, since one prescription is enough to answer your research question(s)?