In addition to training systems, soil effects, and rootstocks, productivity is one of the most important parameters when comparing apple cultivars to determine ultimate profitability. Pomologists have long struggled with the question of how best to determine productivity when comparing trees of different size, such as those encountered in rootstock evaluations.
W. Autio, D. Greene, and W. Lord presented a comparison of several apple rootstocks with 'McIntosh' as the scion cultivar. They also evaluated and compared different approaches to measuring apple tree productivity. See this article titled "Performance of `McIntosh' Apple Trees on Seven Rootstocks and a Comparison of Methods of Productivity Assessment".
Dr. Wesley Autio, University of Massachusetts Amherst