MGL has analyzed the correlation between minor league and major league performances in consecutive years and draws comparisons to the same results for major leaguers only. The minor league stats are converted to major league equivalents for the analysis.
The bottom line is that minor league stats aren’t as strongly correlated with the next year as major league stats, but the correlation is still pretty strong. For major leaguers, a typical year-to-year correlation for basic battings ratios (OBP, SLG) is around .5 or .6. For minor leaguers promoted to the majors, it’s around .3 or .4. A question in my mind is whether selection bias is inflating the minors-to-majors correlations.