First, we observe that, perhaps contrary to expectation, the proportion of the highest values of PMI (the area to the right of PMI=4 in Figure 2) does not yield a consistent correlation with essay scores.
inasmuch as highest PMI values tend to capture multi-word expressions (South and Africa; Merill and Lynch), morphological variants (bids and bidding), or synonyms (mergers and takeovers), their proportion in word type pairs does not seem to give a clear signal regarding the quality of writing.