Age, height, weight, and build (both torso vs legs and stocky vs skinny) are the possible factors I came up with, but not only do I not know if/how much they do, some I'm not even sure which way some would sway the risk, if that makes sense. Basically, I'm thinking if I have a Parkway SG and another booster available for 2 kids (and the fit looks good for both kids in both boosters), how would I know which kid is more likely to benefit from the SG? I think the answer is going to be "we don't know" but I'd like to hear people's thoughts even if that is the case. Thanks.