Under-stemming

...counts words that should be transformed to the same group but are not. For instance, running is transformed to run, but ran, instead of being also transformed to run, is transformed to ran. This causes a single information to be spread over various stems (run and ran in our small example), that is, one object of a user's search is represented as two different stems in the index. Clearly, this situation decreases the overall recall in a system that uses such stemmer.