The subjectivity and non-probability based nature of unit selection (., selecting people, cases/organisations, etc.) in purposive sampling means that it can be difficult to defend the representativeness of the sample. In other words, it can be difficult to convince the reader that the judgement you used to select units to study was appropriate. For this reason, it can also be difficult to convince the reader that research using purposive sampling achieved theoretical/analytic/logical generalisation . After all, if different units had been selected, would the results and any generalisations have been the same?
At the undergraduate and master's dissertation level, you will often focus on just two variables: an independent and a dependent variable ; or sometimes, a second or third independent and/or dependent variable [see the article: Types of variables ]. Only in a minority of cases are you likely to examine a large number of variables at once. However, just because you are only focusing on a small number of variables, this does not mean that these are the only variables that relate to the research you are performing. In this respect, an extraneous variable refers to any variables that you are not intentionally studying (or cannot study, perhaps because of reasons of cost or difficulty). Rather than there being just a few of these extraneous variables, there are likely to be hundreds or even thousands. In other words, it is impossible to avoid extraneous variables.