> General chance of being in either group: 50% (10 people with autism, 10 with ADHD, 20 total). This is wrong. With your numbers ("W (9), X (1), Y (9), Z (1)")

> General chance of being in either group: 50% (10 people with autism, 10 with ADHD, 20 total). This is wrong. With your numbers ("W (9), X (1), Y (9), Z (1)"), 18 people would have ADHD, not 10.