Statistical deviation denotes the percent of variation that a score deviates from the statistical norm. Statistical deviation refers to how large a group of a selected population sample differs from the remainder of the same population.
For instance, imagine taking a sandwich survey of a population of 100 1st graders at an elementary school. Next, find out how many of them like peanut butter sandwiches. Say, out of this 100 1st graders 85 students like peanut butter and 15 don't. This would mean that 15% of this sample deviated from the norm of students who like peanut butter sandwiches.