Let's assume I want to conduct a survey. To find test subjects, I post an ad in a local newspaper, one on a locally relevant website, distribute flyers in front of the train station, and have managed to get one complete class from a local school to partake.
Due to the different recruitment methods, each will probably have drawn quite different samples from the population: the local newspaper is mostly read by older people with medium to high education; the institute website is mostly visited by students and researchers; the flyer is given to commuters; then there is that group of pupils, all of the same age and education level.
In my test, I ask my test subjects to specify how they knew of the test:
- ad in newspaper
- ad on website
- they were tested in class
So I can group my data according to the recruitment method, if I want to.
Is this all one sample? Or have I drawn four samples?
And if this is one sample, how then do you draw two or more samples?