What kind of sampling should be done when there is a small group of patients, easily available?

Random sampling (simple random sampling) - It is a technique whereby each sampling unit has the same probability of being selected.  - Random sampling is done, when there is a small group of patients.  Multiphase sampling:  - This is used to take basic data from a large sample and details from a sub-sample. Multistage sampling - The first stage is to select the groups or clusters. Then, subsamples are taken in as many subsequent stages as necessary to obtain the desired sample size.  Eg:  1st stage: Choice of states within countries.  2nd stage: Choice of towns within each state. 3rd stage: Choice of neighborhoods within each town. Soben Peter  Essentials of preventive and community dentistry  4th edition