You right, productivity boost can caused by knowing that your productivity is tracked and surveyed...

However, here in New Zealand one of the most important is the work life balance. Everything is about being happy and enjoy your life.

For this reason, it is quite common the 37 hours workweeks, flexible hours, working from home at least one day a week and go for coffee or lunch when you want...

super cool country actually ;)

I was also thinking that if the employees knew that it was a trial run, they might work harder than they otherwise would in the hopes that if they did well productivity-wise, they'd get to keep the 4-day work week after the trial was over. Obviously if the company recorded productivity losses, they'd scrap it and move on.

A better test would be to announce a 4-day work week, and tell people it's a permanent change. (Of course, if people's productivity drops, and then you go back to 5 days, morale will take a hit.)