On the 8 November 2015 Slate Star Codex open thread I asked if anyone had questions that could be answered via a survey of the population. Gwern said he wished to know "what fraction of cats respond to catnip", claiming to have a cat that was non reactive. Catnip is a plant known for being very attractive to cats and causing them to act briefly psychotic. Various statistics can be found floating around that internet, Scientific American claims 70 to 80 percent of cats are susceptible and PetMD claims only about 50% are susceptible. These statistics are all uncited and likely the root of the grape vine is just someones best guess.
So I developed a survey that asked respondents about their cat ownership and experiences with using catnip on their cat. It was distributed on this website. After taking a personality test users were asked if they would be willing to complete an additional short research survey before they viewed their results (about half agree to).
Of 5624 respondents, 2340 (41.6%) reported owning a cat. This is close to the estimates for percentage of households that own cats (30-35%). Of the 2340 respondents with cats, 1339 (57.2%) reported that they had ever given their cat catnip. Of the 1339 who had given their cat catnip, 183 (13.6%) said it had no effect, 619 (46.2%) said it had some effect, and 537 (40.1%) said it had a crazy effect. It would appear that catnip insensitivity is relatively rare. Although measurement quality is pretty low here as one man's "crazy response" may be another man's "no response".
They survey also included questions about the cat such as its gender, age and weight. None of these were important moderators of reported catnip response.