Question 2.5: Why is it difficult to test for strong form efficiency?

It is hard to test for strong form efficiency directly, i.e. by studying the market’s use of information, because it can always be objected that investors with access to inside information can make abnormal gains. Tests for strong form efficiency are therefore indirect, examining the performance of users of information who may have access to inside information or who have special training for share dealing, such as fund managers.