Before revision:  To provide an understanding of the literature, we discuss each of these areas separately, identify the most common practices and their alternatives , and propose possible future research directions for this interesting and important field .
After revision:  To provide an understanding of the literature, we discuss each of these areas separately, identify the most common practices and their alternatives . Conclusions: Smartphones are well-suited for HAR research in the health sciences. For population-level impact, future studies should focus on improving quality of collected data, address missing data, incorporate more diverse participants and activities, relax requirements about phone placement, provide more complete documentation on study participants, and share the source code of the implemented methods and algorithms .