Engaging a representative sample of participants in user research sounds like a good idea but it is flawed. It requires lots of participants, does not work in an agile development environment, stifles innovation and reduces your chances of finding problems in small sample usability tests. When combined with iterative design, theoretical sampling (where theory and data collection move hand in hand) provides a more practical alternative.
Why you don’t need a representative sample in your user research

People coming from a marketing or advertising background often have trouble believing the effectiveness of small sample sizes.