title: Crowdsourcing a normative natural language dataset: a comparison of Amazon Mechanical Turk and in-lab data collection.
abstract: BACKGROUND
  Crowdsourcing has become a valuable method for collecting medical research data. This approach, recruiting through open calls on the Web, is particularly useful for assembling large normative datasets. However, it is not known how natural language datasets collected over the Web differ from those collected under controlled laboratory conditions.
  
  
  OBJECTIVE
  To compare the natural language responses obtained from a crowdsourced sample of participants with responses collected in a conventional laboratory setting from participants recruited according to specific age and gender criteria.
  
  
  METHODS
  We collected natural language descriptions of 200 half-minute movie clips, from Amazon Mechanical Turk workers (crowdsourced) and 60 participants recruited from the community (lab-sourced). Crowdsourced participants responded to as many clips as they wanted and typed their responses, whereas lab-sourced participants gave spoken responses to 40 clips, and their responses were transcribed. The content of the responses was evaluated using a take-one-out procedure, which compared responses to other responses to the same clip and to other clips, with a comparison of the average number of shared words.
  
  
  RESULTS
  In contrast to the 13 months of recruiting that was required to collect normative data from 60 lab-sourced participants (with specific demographic characteristics), only 34 days were needed to collect normative data from 99 crowdsourced participants (contributing a median of 22 responses). The majority of crowdsourced workers were female, and the median age was 35 years, lower than the lab-sourced median of 62 years but similar to the median age of the US population. The responses contributed by the crowdsourced participants were longer on average, that is, 33 words compared to 28 words (P<.001), and they used a less varied vocabulary. However, there was strong similarity in the words used to describe a particular clip between the two datasets, as a cross-dataset count of shared words showed (P<.001). Within both datasets, responses contained substantial relevant content, with more words in common with responses to the same clip than to other clips (P<.001). There was evidence that responses from female and older crowdsourced participants had more shared words (P=.004 and .01 respectively), whereas younger participants had higher numbers of shared words in the lab-sourced population (P=.01).
  
  
  CONCLUSIONS
  Crowdsourcing is an effective approach to quickly and economically collect a large reliable dataset of normative natural language responses.
authors.affiliation: Schepens Eye Research Institute, 20 Staniford Street, Boston, MA 02114, USA. daniel_saunders@meei.harvard.edu
authors.forename: Daniel R
authors.initials: DR
authors.lastname: Saunders
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Bex
authors.forename: Russell L
authors.initials: RL
authors.lastname: Woods
mesh_terms: D000328:Adult; D000368:Aged; D063045:Crowdsourcing; D003625:Data Collection / Q000379:methods*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D011381:Programming Languages; D014481:United States; D055815:Young Adult
keywords: Internet; crowdsourcing; free recall; web