Construct validity in the Trail Making Test: what makes Part B harder?

The Trail Making Test (TMT) is primarily a test of motor speed and visual attention. In Trail Making, Part A, the subject's task is to quickly draw lines on a page connecting 25 consecutive numbers. In Part B, the subject must draw the lines alternating between numbers and letters. To determine what makes Part B harder than Part A, variations of the standard Trail Making Test were assessed. Forty college students (20 male, 20 female) were given four forms of the Trail Making Test. The results show that Trail Making, Part B with just numbers took longer to complete than the standard Part A with numbers. Part B is 56 cm longer and has more visually interfering stimuli than Part A. These results indicate that Part B is more difficult than Part A not only because it is a more difficult cognitive task, but also because of its increased demands in motor speed and visual search.