Posted by Martin on .
I have to sample 300 persons. How do I go about doing this and how do I know that the population I have chosen has the characteristics that I am looking for?

Statistics 
PsyDAG,
The population is the group you want to gain information about. The sample is the 300 persons, a portion of that population.
What population concerns you? Children? Adults? College students? NFL players? Politicians? Nurses?
What do you want to find out about this population?
How are you sampling them? Written questionnaires mailed or given in person? Blood samples? Behavioral samples?
Regardless of the population, the sample needs to be random — or representative of the population.
Once you have defined what you are studying, you might want to repost your question.
I hope this helps. Thanks for asking.