title: Impact of length or relevance of questionnaires on attrition in online trials: randomized controlled trial.
abstract: 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).
  
  
  OBJECTIVE
  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.
  
  
  BACKGROUND
  ISRCTN Register 31070347; http://www.controlled-trials.com/ISRCTN31070347/31070347 Archived by WebCite at (http://www.webcitation.org/62cpeyYaY).
authors.affiliation: Faculty of Public Health & Policy, London School of Hygiene & Tropical Medicine, London, United Kingdom. Jim.McCambridge@lshtm.ac.uk
authors.forename: Jim
authors.initials: J
authors.lastname: McCambridge
authors.forename: Eleftheria
authors.initials: E
authors.lastname: Kalaitzaki
authors.forename: Ian R
authors.initials: IR
authors.lastname: White
authors.forename: Zarnie
authors.initials: Z
authors.lastname: Khadjesari
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Murray
authors.forename: Stuart
authors.initials: S
authors.lastname: Linke
authors.forename: Simon G
authors.initials: SG
authors.lastname: Thompson
authors.forename: Christine
authors.initials: C
authors.lastname: Godfrey
authors.forename: Paul
authors.initials: P
authors.lastname: Wallace
mesh_terms: D000328:Adult; D000428:Alcohol Drinking / Q000523:psychology / Q000628:therapy; D005260:Female; D006801:Humans; D020407:Internet; D008297:Male; D008875:Middle Aged; D010352:Patient Dropouts / Q000706:statistics & numerical data*; D010865:Pilot Projects; D011795:Surveys and Questionnaires
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