What is considered an error in the sustained attention to response task (SART)? The traditional SART task requires key presses to all digits appearing on the screen. However, the key press should be withheld if the digit is a 3

When aggregating the collected data, what is considered an error?

Is it simply the number of times the key was pressed when a 3 is on the screen? Or does it also include incorrectly withheld key presses (i.e. when the digit is anything other than 3)?
Both are errors indeed. Pressing when a 3 is shown is called a false alarm, and not pressing when a three is shown is a miss. Correct responses are (1) hits (pressing when anything but a three is shown) or (2) correct rejections (not pressing with a three). 



These classifications are used in signal detection theory (Wiki). More specifically, they are used to calculate sensitivity and bias.

Sensitivity, or d-prime (see also other questions on CogSci, e.g. Calculating d-prime), is the metric for how well one can distinguish targets from distractors. Bias, on the other hand, is:


  the extent to which one response is more probable than another (Wiki)