This is great in that it shows that people who volunteer to work from home end up being more productive at home. However, knowing that you are part of a study that is measuring how effective people are at home and in the office and being someone who volunteered to work from home, there are clearly incentives for those assigned to work from home to work harder and for those assigned to work in the office to work less hard.

Except the people who worked in the office also volunteered to stay, so why wouldn't they work harder too so they don't have to start working at home when they don't want to?

Sorry, that is just pertaining to the first part of the study. After seeing those results they did indeed allow all who wanted to work from home and saw an increase in their productivity.

That is a really important detail I must say.