So you know your study’s goals, you have your research questions, and you have the perfect data analysis in mind. Everything may be in the works, but there are some very important details to consider that can be easily overlooked! Before your Institutional Review Board (IRB) can give you the green light to start data collection, and even before any of your committee members can tell whether your research is feasible, you need to know how you will be getting your data. That sounds pretty straightforward, but sampling (the method of recruiting participants to use for their sweet, sweet data) takes more consideration than you might think!
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One of the first steps to deciding on a sampling strategy is to know whether your study is going to be qualitative (mainly using interviews or observations) or quantitative (using numerical data from surveys, census data, etc.). Qualitative research is usually intended to hone in on a specific focus, while quantitative research is usually intended to be generalizable to a broader sense. This distinction is usually what draws the line between purposeful and random sampling. Based on intent of your research, you can look at both forms of sampling like this:
Specific focus: If you are interested in one tidepool from the ocean, you can take a single vial of the water and be pretty sure that whatever you find will be applicable to that pool. However, saying that what you find will hold true for the entire ocean would be a long shot. The purpose of this tight focus would be to learn lots of information about a very specific thing.
General focus: Using the same analogy, a researcher would not try to learn about global water temperatures by taking a vial from one tide pool. The best way to do this would be to take to the sea and randomly collect vials from many different areas to get a general idea of what the average temperature is. The randomness here helps to even out confounding things like water currents, random temperature spikes, distance from the equator, and the like.
Later on, we will talk a little more about the different methods within the overall schools of random and purposeful sampling. Stay tuned!
In 50 Word or Less: If you are conducting a qualitative study, you probably want to use purposeful sampling. If your study is quantitative, random sampling is usually best. However, this is just the first step in the decision-making process, so it does not always hold true.