Why does every little kid, who doesn’t get the answer they want from Mom, then go and ask Dad or Grandma or Auntie hoping to get a different response? Because kids know that the answer you get to your question depends on who you ask!!
In research it’s the same. Sometimes the answer you get depends on who you ask. Thus, it’s not only important to write out a well-formulated research question, but you need to ask the right persons! Unlike little children, however, you need to ask a representative sample that will give you an accurate scientific answer–not just an answer that matches your preconceived notions of the answer you want. The answer you need is not necessarily the one you want…or expect.
What research sample will give you a more accurate, scientific answer? It’s a sample that represents the population of interest. NO sampling method guarantees a representative sample from the population, but some sort of random selection of participants from the population is likely to be best. Sometimes, however, it’s not possible or affordable to do random sampling and so researchers pick their sample using a different sampling method.
A concise 5 minute summary of some sampling methods is here: https://www.youtube.com/watch?v=be9e-Q-jC-0
Remember that the BEST samples are REPRESENTATIVE. That may or may not mean they are randomly selected.
Whatever sampling method is used, be clear yourself about the strengths and weaknesses of that sampling method. Report your sampling method in the procedures section of your final report. That way your readers can tell whether you asked subjects who were the equivalent of “Mom,” “Dad,” or both.