How the CDC study handled biases in observational data – Big Data, Plainly Spoken (aka Numbers Rule Your World)

This bias originated from the drastic decline in infection rates during the first quarter of 2021. Upon vaccination, an individual migrates from the unvaccinated group to the vaccinated group. If we tally up the person-hours for the unvaccinated, they will skew towards the start of the study compared to those for the vaccinated group.