Is it for a quantitative or a qualitative study? If it's quant, then questions like the one TreeofLife suggested would work very well (you could also ask them to rate influence on a 1-9 scale ranging from negative to positive influence, as another example). If it's a qualitative study and you're asking more open-ended questions for them to write their own answers, try to ask them in such a way that you're not suggesting positive or negative influence (like the example Lizzie gave).