Academic and industry evidence have shown that excessive questionnaire length (i.e. more than 15 minutes) has a negative effect on data quality, as measured by data reliability, variance and validity (Galesic and Bosnjak, 2009; Marcus, Bonsnak, Linder, Pilischenko and Schütz, 2007).

These studies also present evidence that the completion rates for online questionnaires drops off significantly as the length of time taken to complete the questionnaire increases (i.e. the completion rate for a 10 minute survey was 67.5%, while a 30 minute survey had a completion rate of 18.6% (Marcus, Bonsnak, et al., 2007).

Aside from questionnaire length, other factors have an impact on response rates, completion rates and data quality.