title: Using multiple imputations to accommodate time-outs in online interventions.
abstract: BACKGROUND
  Accurately estimating the period of time that individuals are exposed to online intervention content is important for understanding program engagement. This can be calculated from time-stamped data reflecting navigation to and from individual webpages. Prolonged periods of inactivity are commonly handled with a time-out feature and assigned a prespecified exposure duration. Unfortunately, this practice can lead to biased results describing program exposure.
  
  
  OBJECTIVE
  The aim of the study was to describe how multiple imputations can be used to better account for the time spent viewing webpages that result in a prolonged period of inactivity or a time-out.
  
  
  METHODS
  To illustrate this method, we present data on time-outs collected from the Q(2) randomized smoking cessation trial. For this analysis, we evaluate the effects on intervention exposure of receiving content written in a prescriptive versus motivational tone. Using multiple imputations, we created five complete datasets in which the time spent viewing webpages that resulted in a time-out were replaced with values estimated with imputation models. We calculated standard errors using Rubin's formulas to account for the variability due to the imputations. We also illustrate how current methods of accounting for time-outs (excluding timed-out page views or assigning an arbitrary viewing time) can influence conclusions about participant engagement.
  
  
  RESULTS
  A total of 63.00% (1175/1865) of participants accessed the online intervention in the Q(2) trial. Of the 6592 unique page views, 683 (10.36%, 683/6592) resulted in a time-out. The median time spent viewing webpages that did not result in a time-out was 1.07 minutes. Assuming participants did not spend any time viewing a webpage that resulted in a time-out, no difference between the two message tones was observed (ratio of mean time online: 0.87, 95% CI 0.75-1.02). Assigning 30 minutes of viewing time to all page views that resulted in a time-out concludes that participants who received content in a motivational tone spent less time viewing content (ratio of mean time online: 0.86, 95% CI 0.77-0.98) than those participants who received content in a prescriptive tone. Using multiple imputations to account for time-outs concludes that there is no difference in participant engagement between the two message tones (ratio of mean time online: 0.87; 95% CI 0.75-1.01).
  
  
  CONCLUSIONS
  The analytic technique chosen can significantly affect conclusions about online intervention engagement. We propose a standardized methodology in which time spent viewing webpages that result in a time-out is treated as missing information and corrected with multiple imputations.
  
  
  BACKGROUND
  Clinicaltrials.gov NCT00992264; http://clinicaltrials.gov/ct2/show/NCT00992264 (Archived by WebCite at http://www.webcitation.org/6Kw5m8EkP).
authors.affiliation: Group Heatlh Research Institute, Biostatistics Unit, Seattle, WA, United States. shortreed.s@ghc.org.
authors.forename: Susan M
authors.initials: SM
authors.lastname: Shortreed
authors.forename: Andy
authors.initials: A
authors.lastname: Bogart
authors.forename: Jennifer B
authors.initials: JB
authors.lastname: McClure
mesh_terms: D006801:Humans; D020407:Internet; D009042:Motivation; D016540:Smoking Cessation / Q000379:methods*; D012944:Social Support
keywords: Internet; automatic time-out; behavioral research; engagement; multiple imputations; online interventions; smoking cessation; time spent online; utilization