Yea, position and stacks will be a major factor. The number of hands is also important. if its early stage and he has 31/25 on 12 hands, that may not mean as much as it looks like. He may just be getting good cards, and his PFR looks good against his VPIP. He may just be a LAG and bad, its hard to say yet. If he is 34/0 on 12 hands, that will make fishy bells go off, but still, not many hands. On the other hand if he is 0/0 that is more of a read with few hands. The more hands, the more you can believe the stats.
You need a fair number of hands before you can really put anyone on a hand, but not a ridiculous amount. 20-25 should begin to show a pattern (but still can mean anything, so it only a small hint), and from 75-80, you should have an idea of how loose or tight they are, at least on your table. With not too many hands, you want to think about how the table is playing as well, as this may have an influence over how they play. Of course blinds, position and stacks play a big role as well. As you yourself get more hands under your belt, it will become clearer how to interpret the stats.
I play a lot of the same players now in SnGs, and have 4-600 hands on a few, and it is really amazing how a 15/11 over all can play 37/28 in a particular game. Things like that are particularly important to note, as well as noting the table dynamics as to why he is so aggressive at this table. The 35-40/? + players over many hands tend to always be 35-40(or more)/?+ on any table. (? cause their PF raises are usually either also high or non existent.) I get the occasional 65/?+ player, its always a treat.........