Amoung the many subgenres of mystery stories, the Whodunnit, focuses on who committed the crime. Obviously. The writer of these mysteries can go one of two ways: Fair Play or Clueless. Fair Play mysteries can be read, acted out as in mystery parties or even used in contests as a fancy type of 'skill testing question'.
In the 'fair play' catagory, the author gives the reader all of the clues necessary to solve the mystery. Sometimes there is an element of participation as the reader races against the mind of the detective to get to the solution first.
In the Clueless Mystery, the reader's only hope of solving the thing is a 'lucky guess' because they are not given all the information required.