So those surveys we get a pop up for in game. They have questions that presuppose we've obtained each of the monsters (eg. How likely would you get this monster again?). And the way the survey is, it has to be answered. Why not an option for "I didn't get this monster"? Or questions like "Why didn't you get this monster?" (answer: I couldn't afford it!).
Just saying. Whatever results you're getting are going to be skewed based on how these questions, and answers, are worded.