[thaumasiotes]

The reason people measure first year GPA is that, in the past, all freshmen at a college took the same set of classes. If you measure 3rd year GPA, you get confounded by the difference between physics students and French students.

Obviously, the worth of the metric goes down over time as first-year curricula differentiate from each other.

> But is GPA the best measure, especially first year?

No. For example, SAT score is a better single measure than GPA is. But you can't use that to check the validity of SAT scores.