Test

I have started to make my own quizzes and test. Every time I have used a language arts test from the book my students completely bomb it. The directions are confusing and some of the answers are hard for me to figure out. I do not understand this. There was even a question on one test that had nothing to do with the concept it was supposed to be measuring. Do you think it is better to make your own test or use test from the books. I think it is good to use both. I actually use some questions from the book test, when creating my own test. I just make them more kid friendly.