Mystery stories often feature a brilliant detective and the detective' s dull companion  Clues are presented in the story, and the companion wrongly infers an inaccurate solution to the mystery using the same clues that the detective uses to deduce the correct solution
Thus, the author' s strategy of including the dull companion gives readers a chance to solve the mystery while also diverting them from the correct solution
Some mystery stories give readers enough clues to infer the correct solution to the mystery.