Impact of length or relevance of questionnaires on attrition in online trials: Randomized controlled trial

Background: there has been limited study of factors influencing response rates and attrition in online research. online experiments were nested within the pilot (study 1,n = 3780) and main trial (study 2,n = 2667) phases of an evaluation of a web-based intervention for hazardous drinkers: the down your drink randomized controlled trial (dyd-rct). objectives: the objective was to determine whether differences in the length and relevance of questionnaires can impact upon loss to follow-up in online trials. methods: a randomized controlled trial design was used. all participants who consented to enter dyd-rct and completed the primary outcome questionnaires were randomized to complete one of four secondary outcome questionnaires at baseline and at follow-up. these questionnaires varied in length (additional 23 or 34 versus 10 items) and relevance (alcohol problems versus mental health). the outcome measure was the proportion of participants who completed follow-up at each of two follow-up intervals: study 1 after 1 and 3 months and study 2 after 3 and 12 months. results: at all four follow-up intervals there were no significant effects of additional questionnaire length on follow-up. randomization to the less relevant questionnaire resulted in significantly lower rates of follow-up in two of the four assessments made (absolute difference of 4%,95% confidence interval [ci] 0%-8%,in both study 1 after 1 month and in study 2 after 12 months). a post hoc pooled analysis across all four follow-up intervals found this effect of marginal statistical significance (unadjusted difference,3%,range 1%-5%,p = .01; difference adjusted for prespecified covariates,3%,range 0%-5%,p = .05). conclusions: apparently minor differences in study design decisions may have a measurable impact on attrition in trials. further investigation is warranted of the impact of the relevance of outcome measures on follow-up rates and,more broadly,of the consequences of what we ask participants to do when we invite them to take part in research studies. © jim mccambridge,eleftheria kalaitzaki,ian r. white,zarnie khadjesari,elizabeth murray,stuart linke,simon g. thompson,christine godfrey,paul wallace.