Prevent null datasets, Quality Control

In a small percentage of cases, users signed up for Fitbit accounts or logged into an old account not associated with a device. Ideally, there should be some way of detecting whether the user has a valid account and developing more advanced exclusion criteria (e.g. exclude mobile-app only use, or those who signed up for an account too recently)

Review the Fitbit account and device acquisition flow and trace how datasets are produced for Mechanical Turk users. Determine how missing-device and other invalid-account cases can be detected, then define exclusion criteria for those cases and verify that invalid users no longer produce null datasets.

Written by the indexing model from the issue text.