title: On the influence of a raffle upon responses to an urban transportation survey in New York City.
abstract: OBJECTIVE
  Determine whether raffling off prizes to increase participation in surveys triggers multiple responses from the same participants.
  
  
  METHODS
  All staff, instructors, and students of Queens College, New York, were invited to a 5 minute online survey about commuting in winter 2007. Entry in the raffle to win one of two iPODs was not conditional upon survey completion.
  
  
  RESULTS
  Participation was 18.7 %, and 3913 eligible responses. Of the 183 (4.7 %) participants with two answers, responses were consistent, with weighted kappas ranging between 0.48 and 1.00, and Pearson r's between 0.81 and 1.0. Kappa's were lower for opinion than for factual questions. The duplicate responses came from people with characteristics similar to the full sample. Commuting modes included car (46 %), bus but no subway (28 %), and bus and subway (21 %). The median commuting time was 90 minutes round-trip, and was greater than 120 minutes for 1/4 of the respondents.
  
  
  CONCLUSIONS
  Participation was low despite the raffle. The small percentage of duplicate answers, maybe prompted by the reminding email, had a negligible impact on the results.
authors.affiliation: School of Earth and Environmental Sciences, Queens College - CUNY, Center for the Biology of Natural Systems, 163-03 Horace Harding Expressway, Flushing, NY 11365, USA. alfredo.morabia@qc.cuny.edu
authors.forename: Alfredo
authors.initials: A
authors.lastname: Morabia
authors.forename: Yan
authors.initials: Y
authors.lastname: Zheng
mesh_terms: D003625:Data Collection; D034742:Electronic Mail; D005260:Female; D006801:Humans; D020407:Internet; D008297:Male; D009042:Motivation; D009519:New York City; D018579:Patient Selection; D012201:Reward; D014186:Transportation / Q000379:methods* / Q000706:statistics & numerical data; D014505:Urban Population