  Accurate estimation of post-click conversion rate is critical for building
recommender systems, which has long been confronted with sample selection bias
and data sparsity issues. Methods in the Entire Space Multi-task Model (ESMM)
family leverage the sequential pattern of user actions, i.e.
$impression\rightarrow click \rightarrow conversion$ to address data sparsity
issue. However, they still fail to ensure the unbiasedness of CVR estimates. In
this paper, we theoretically demonstrate that ESMM suffers from the following
two problems: (1) Inherent Estimation Bias (IEB), where the estimated CVR of
ESMM is inherently higher than the ground truth; (2) Potential Independence
Priority (PIP) for CTCVR estimation, where there is a risk that the ESMM
overlooks the causality from click to conversion. To this end, we devise a
principled approach named Entire Space Counterfactual Multi-task Modelling
(ESCM$^2$), which employs a counterfactual risk miminizer as a regularizer in
ESMM to address both IEB and PIP issues simultaneously. Extensive experiments
on offline datasets and online environments demonstrate that our proposed
ESCM$^2$ can largely mitigate the inherent IEB and PIP issues and achieve
better performance than baseline models.

Artificial Intelligence;Information Retrieval