The method used was to compare, in various ways, the number of documents retrieved by six different engines in response to a carefully selected, constrained query. The findings were surprising. One factor that they had to provide a control for in their analysis was the distinct morphology rules that different engines have and, hence, the different ways in which they treat queries. They did not discuss this aspect of searching in any detail.
All search engines refine their method of indexing frequently.
Therefore, we will update our table from time to time.