Assigning Tasks to Workers based on Historical Data: Online Task Assignment with Two-sided Arrivals

John P. Dickerson, Karthik Abinav Sankararaman, Aravind Srinivasan, Pan Xu

Abstract: Efficient allocation of tasks to workers is a central problem in crowdsourcing. In this paper, we consider a special setting inspired from spatial crowdsourcing platforms where both workers and tasks arrive dynamically. Additionally, we assume all tasks are heterogeneous and each worker-task assignment brings a distinct reward. The natural challenge lies in how to incorporate the uncertainty in the arrivals from both workers and tasks into our online allocation policy such that the total expected rewards are maximized. To attack this challenge, we assume the arrival patterns of worker "types'' and task "types'' are not erratic and can be predicted from historical data. To be more specific, we consider a finite time horizon T and assume in each time-step, a single worker and task are sampled (i.e., "arrive'') from two respective distributions independently, and this sampling process repeats identically and independently for the entire T online time-steps. Our model, called Online Task Assignment with Two-Sided Arrival (OTA-TSA), is a significant generalization of the classical online task assignment where the set of tasks is assumed to be available offline. For the general version of OTA-TSA, we present an optimal non-adaptive algorithm which achieves an online competitive ratio of 0.295. For the special case of OTA-TSA where the reward is a function of just the worker type, we present an improved algorithm (which is adaptive) and achieves a competitive ratio of at least 0.343. On the hardness side, along with showing that the ratio obtained by our non-adaptive algorithm is the best possible among all non-adaptive algorithms, we further show that no (adaptive) algorithm can achieve a ratio better than $0.581 (unconditionally), even for the special case of OTA-TSA with homogenous tasks (i.e., all rewards are same). At the heart of our analysis lies a new technical tool (which is a refined notion of the birth-death process), called the two-stage birth-death process, which may be of independent interest. Finally, we perform numerical experiments on two real-world datasets obtained from crowdsourcing platforms to complement our theoretical results.