A similar problem happened at Harvard Business School. In the mid 80s they found no correlation between GMAT scores and success at the school, so they stopped

A similar problem happened at Harvard Business School. In the mid 80s they found no correlation between GMAT scores and success at the school, so they stopped requiring applicants to submit scores. This was a thermostat-unaware view. The real answer was that they were giving it the right amount of weight. If they were overweighting the GMAT, unqualified people with high GMATs would get in. If they were not underweighting it, the opposite. Approximately 10 years later they did a correlation of student GMATs (which weren't used for admissions purposes) and school success, and found that there was indeed now a correlation. The lesson was that if you're properly managing a factor, you won't see it in the outcome. Or in the thermostat example, if you're properly managing the thermostat, you won't see the changes.