I was in a study at work to find out how much health care really costs the workers versus the employer. For three months I wrote down every penny I spent on any health care cost. I did this for two separate time periods (each three months long). It seemed to me that whenever I kept this cost diary, nothing happened. As soon as the time ended, I ended up paying out-of-pocket for something. Is this kind of study really accurate?

Researchers count on the law of probabilities in studies like this. Most of the people keep a diary that reflects most of their costs most of the time. There’s probably someone else in the study with high costs who stopped spending money when the study started. That person would say the study doesn’t reflect his or her spending at all.

Asking people to keep track for longer periods of time (up to one-year) would probably be more precise. However studies of cost diaries actually show a record for three months is a good sample of what really happens over a full year.