Crowd-sourcing has become a popular means of acquiring labeled data for many tasks where humans are more accurate than computers, such as image tagging, entity resolution, and sentiment analysis. However, due to the time and cost of human labor, solutions that rely solely on crowd-sourcing are often limited to small datasets (i.e., a few thousand items). This paper proposes algorithms for in- tegrating machine learning into crowd-sourced databases in order to combine the accuracy of human labeling with the speed and cost- effectiveness of machine learning classifiers. By using active learn- ing as our optimization strategy for labeling tasks in crowd-sourced databases, we can minimize the number of questions asked to the crowd, allowing crowd-sourced applications to scale (i.e., label much larger datasets at lower costs).

Designing active learning algorithms for a crowd-sourced database poses many practical challenges: such algorithms need to be generic, scalable, and easy to use, even for practitioners who are not machine learning experts. We draw on the theory of nonparametric boot- strap to design, to the best of our knowledge, the first active learning algorithms that meet all these requirements.

Our results, on 3 real-world datasets collected with Amazons Mechanical Turk, and on 15 UCI datasets, show that our methods on average ask 1–2 orders of magnitude fewer questions than the base- line, and 4.5–44× fewer than existing active learning algorithms.