In the 1980’s, Elliot Soloway’s group at Yale began capturing all the programs written by computer science students. The students were writing in a programming language called Pascal [154], a notation for writing computer programs that was explicitly invented to be good for teaching students. Student performance on one particular problem intrigued Elliot and his students [294]. […]
Students were expected to write a program that would accept numbers as input from a user at the keyboard. Thee students had to infer what variables would be needed to solve this problem and the combination of things to keep in mind at each step. Each number would be added to a running total of all the numbers read, and a count would be kept of the number of input numbers, so that the average could be printed at the end. If a negative number was input, that should be considered a mistake. The program should not add the number to the total, and the count should not be incremented, but the program should go on to read more numbers. If the input number was 99999, then the program should stop reading numbers, and shouldn’t add that number to the total, and shouldn’t increment the count. Then, print out the average.
This doesn’t seem like an unreasonable task for a Yale Computer Science student. This problem wasn’t computing the next prime number after 1000, or computing the 10,000th digit of pi, or writing a program for Tetris or Angry Birds. However, this was a surprisingly difficult task.

• When Elliot’s group gave this task to the first-term computer science students (about week 12 of a 16-week semester), only 14% of the students got it right. Getting the programming language wrong wasn’t the issue - syntax errors were ignored in this analysis. Most often students had trouble not adding in the negative or end numbers, or they didn’t consider troublesome cases, e.g., what if only negative numbers were input?
• Elliot’s group also gave this task to students in their second CS course, in the same 12th week of a 16-week semester. Now, 36% of the students got it right.
• Finally, Elliot’s group gave the task to students in an advanced Systems Programming course, where everyone was a Junior or Senior (3rd or 4th year). 69% of the students got it right. Not 100%, for a problem that seems reasonable to give to first-year students.