This was my thought too. The findings are written as "unit tests cause X" when really what is shown is "unit tests are associated with X". Another explanation

This was my thought too. The findings are written as "unit tests cause X" when really what is shown is "unit tests are associated with X". Another explanation for the results could be common cause: Maybe packages with more unit tests are also professionally developed, or written by people interested in software methodology, or are older; and those aspects are also associated with method length/decomposition patterns/etc.