2-Subjects might perform differently because they know they were (or being) studied.
This is unclear because of the tenses- if you are talking about the study that has been done, then use 'might have behaved... were', if it's about a possible study in the future, then use 'might behave...are'.
3- Population studied may not be representing population of physicians in the different places and current generation of physician or physicians practice in the hospitals.
The population studied may not be representative of the population of physicians in other places and the current generation of physicians or physicians practising in hospitals.
I used 'other' and no article because I didn't think you were referring to specific places, and dropped the article before 'hospitals' for the same reason. if you are being specific, then please restore them to their old places.
4- prescribe drugs different from other physician
prescribe different drugs or prescribe drugs differently? Also, 'physicians'
5- same as four
Student or Learner