For an item analysis, there were 100 students in the upper g…

For an item analysis, there were 100 students in the upper group and 100 students in the lower  group (200 total).  The data are the percentage in each group choosing each option, with correct answers marked by an asterisk (*):                      (a)       (b)       (c)       (d) upper       10       03     77*    10 lower       21        29       11*     39 What is the discrimination index for this item (as a proportion)?