The subjects had limited time and some were unfamiliar with the applications. One subject used up 10 of his 15 minutes struggling with MacDraw to learn how to shorten lines manually. He asked me if I could answer questions about MacPaint, but the study format was that all subjects would work unaided so I could not clear up the fact that he was working with MacDraw, not MacPaint. One way to explain many of the errors is the fact that the subjects had no programming tools. Few people write out programs on paper and expect them to be perfect.
Testing is part of most program development.
When I asked the subjects to explain the program they correctly explained how it would change the setting to 100. Then when I asked what the program would do next, they quickly saw that the program would repeatedly click on the popup menu, even though 25 was no longer showing. Then they quickly realized what the correct pattern should be. This is the sort of error that would be obvious upon running the program, and that would have an easy solution. Errors in the other 10 programs suggest that pixel data access and the programs in which it will be used have an inherent complexity that must be addressed.