base (one's) opinion on
To found or root one's view of something or someone on a particular idea or occurrence. Unfortunately, I think she based her opinion of me entirely on one wild night out together. I'm basing my opinion on the facts, and nothing else.
base one's opinion on something
to make a judgment or form an opinion from something. You must not base your opinion on one bad experience. I base my opinion on many years of studying the problem.