I have no experience with -afinils, but it seems to me that there will surely be cases of people who have tried only brand-name (or, alternatively, only generic) -afinil, and therefore cannot accurately respond to the question

In general, do you find brand-name -afinils more effective than generics?

With yes, no, or the same. The correct answer would be "I don't know". If I were taking this survey, I would skip that question rather than try to guess which answer you wanted in that case. But if I were designing the survey, I would go with ChristianKl's suggestion.