One of the things that we know about sampling is that it's important for samples to be representative of the population, also known as a representative sample. What we mean by that is when we take our sample--which is a subset of a larger population--we want this sample to behave just like the population would if we sampled them all.
We want the sample to behave as similar to the population as possible so that when we calculate statistics from our data, the statistics are as accurate about the population as they can be.
The goal is to generalize what is found in the sample and apply it to the people outside of the box, or the population.
The two methods analyzed in this tutorial have major flaws--these two designs do not result in representative samples. They are conducted often, so it’s important for you to recognize them.
The people he asks might be representative of the population, but they might not. They all simply happen to be at the same place at the same time. This means they might have some similarities that could make them not representative of the larger population. The risk of them not representing the group/population at large is too high.
EXAMPLEIf you ask people about their spending habits, and they all happen to be shopping in the headphones section, that probably means they have similar ideas about how they should spend their money.
EXAMPLEFocus groups are a common example of self-selected samples.
Participants who feel very strongly about the subject at hand are likely to be the volunteer for the self-selected sample. On the other end of the spectrum, participants may be compensated for their time and may simply tell the interviewer what they want to hear.
EXAMPLEIf your focus group is about politics, you might get only the very, very liberal people or the very, very conservative people. You might get the most extreme viewpoints but none of the viewpoints in the middle. Or, there are also a lot of people who are ambivalent about politics. They don't really care, but they want to get paid if this is a sample that offers compensation or another type of reward like free lunch.