# Find the mistake exercise

Find the mistake is a simple, easy to prepare activity. I’ve used it a number of times but definitely not as much as I should despite it being very effective.

During a lecture I distribute photocopies of mistakes from student exam scripts and instruct my current students to locate all the errors. Students usually look for straightforward arithmetical or algebraic mistakes such as negative sign errors and so I usually warn them to look for deeper conceptual mistakes. After a suitable period of time I ask the class what they have found.

An example for complex analysis is here.

Find-the-mistake has two main benefits. One, the students are active and forced to apply their knowledge. In answering a question or solving a problem students often have to construct a multi-step argument and a single small mistake can lead to messing up the whole exercise. When looking for mistakes they don’t have that problem — missing one mistake won’t prevent them finding the next one. Furthermore, a common student complaint is that they can’t get started when solving a problem. Here’s one activity where the barrier to starting successfully is lower than usual.

The second benefit is that they learn what the common mistakes are. This is a perfect example of “Show, don’t tell” applied to education. Obviously, I can tell them what the common mistakes are but they won’t necessarily absorb that. This exercise shows them. Furthermore, using the same mistake from a number of scripts helps the students spot for themselves the common mistakes.

Generating find-the-mistake exercises is easy. When marking exams I like to take a note of the common — or interesting — errors made and photocopy these later for this activity. Student exam scripts are usually kept for a few years so if you are new to a course, you don’t need to wait until the exam is taken, just get them out of storage.

It is important to anonymise the examples used and when going through the answers in class it’s a good idea to not make fun of the mistakes made. For students it’s only a small jump from the lecturer is making fun of other students’ mistakes to the lecturer is going to make fun of my mistakes. And that’s not good!