This is a graph from my app that is very misleading:
Students take a test, then they learn the material, then they take the test again. We want to show the improvements in the students who have pre- and post-test data.
This graph shows the results, but it's very misleading because in this case, only one student (of 27) actually took the post test, and he got a 100%. But I think most people looking at this would think that it means that the whole class got 100%.
So: how can I present this information, but also including the fact that only 17 kids took the pre-test, and only 1 of them took the post-test? The graph thus needs to show three groups:
- Kids who took neither test and have no data, really, other than to establish the population size
- Kids who took the pre-test and how they scored on it
- Kids who took the pre-test AND post-test and how they scored on each, in such a way that the comparison is indicated