How One Microtask Affects Another

Microtask platforms are becoming commonplace tools for performing human research, producing gold-standard data, and annotating large datasets. These platforms connect requesters (researchers or companies) with large populations (crowds) of workers, who perform small tasks, typically taking less than five minutes each. A topic of ongoing research concerns the design of tasks that elicit high quality annotations. Here we identify a seemingly banal feature of nearly all crowdsourcing workflows that profoundly impacts workers' responses. Microtask assignments typically consist of a sequence of tasks sharing a common format (e.g., circle galaxies in an image). Using image-labeling, a canonical microtask format, we show that earlier tasks can have a strong influence on responses to later tasks, shifting the distribution of future responses by 30-50% (total variational distance). Specifically, prior tasks influence the content that workers focus on, as well as the richness and specialization of responses. We call this phenomenon intertask effects. We compare intertask effects to framing, effected by stating the requester's research interest, and find that intertask effects are on par or stronger. If uncontrolled, intertask effects could be a source of systematic bias, but our results suggest that, with appropriate task design, they might be leveraged to hone worker focus and acuity, helping to elicit reproducible, expert-level judgments. Intertask effects are a crucial aspect of human computation that should be considered in the design of any crowdsourced study.