Does practicing writing essays make you better at writing essays?

We’ve all written innumerable essays from middle school onwards. All teachers appear convinced that this will improve students’ essay writing skills. However, when trying to find out what this belief is based on, I found only a single study where some students were assigned practice essays and others were not, and their results on an essay-writing exam were compared. The result of the study, in the authors’ words, was as follows.

As discussed in the previous Part, we found that, on average, students who received the writing interventions had higher average raw scores on each of the essay questions. Although this point spread did not rise to the level of statistical significance, we found between a two and three point raw score difference (out of a possible thirty raw score points on the first question and thirty-three raw score points on the second question). [My bold]

Effectively the authors seem to have found no effect. Does anyone have any more data on this?

Edit:
I can amuse you further with the following quote from the article. It seems I am not the only one having trouble finding support for the claim that essay writing practice improves performance.

For example, we have found no work attempting to measure empirically the impact of practice writing exercises, combined with feedback, on student essay exam performance.