How do they gather data? Officially reported by employers or self-reported? Both of them can have huge biases, from both intended and unintended reasons. (Plus

How do they gather data? Officially reported by employers or self-reported? Both of them can have huge biases, from both intended and unintended reasons. (Plus, in office job often there is no clear cut. People can be Facebooking (or HNing :)) in their working hours... or on top of 8h/day it may be quite a few hours of emails and stuff people are expected to do remotely.) And what is the main impact on the total number of working hours? (Part-time jobs? Holidays? Parental leaves? Sick leaves?)