One strategy used to increase response rates when using mail questionnaires is to prolong the period of data collection. This paper examines the consequences of such a strategy on the Australian Election Studies (1996–2004). The findings suggest that such a strategy has both positive and negative consequences on the overall quality of the survey. On the one hand, prolonging the period of data collection decreases the non-response error by providing a somewhat better representation of hard-to-reach groups of respondents such as the young, full-time employed, and those disinterested in politics. On the other hand, prolonging the period of data collection increases the risk of measurement error. The evidence suggests that post-election events significantly impact on respondents' political opinions. More specifically, the longer respondents wait to return their questionnaire after the elections the more positive their opinions of winning leaders, the more negative their opinions of defeated leaders and the more distant they feel with the policies of defeated parties. Australian investigators thus face a dilemma in deciding when to stop the fieldwork period for their mail election surveys: they must choose between non-response error and measurement error.