The response rate is normally applied to questionnaires or surveys and is the percentage of people in the sample who actually respond. One problem with a postal questionnaire for example is that it relies on people putting their response back in the post. The problem with a poor response rate is that it makes the sample less representative as not only is the sample size reduced but any attempts the researcher might have made to make the sample representative (such as using stratified random sampling) are undermined and in an unpredictable way. Researchers explore ways to try and increase response rate such as offering an incentive to respond (such as a prize draw) but that then adds to the expense of the research.