It will affect the stat from 1-31 points(and no Pokemon can have 0 IVS on one stat)EVs can give you up to a total of 63 points in two stats(or 126 spread over however many stats, but with the 126 being the MAX you can get.) IVs can only boost you from 1 to 31 in each stat, but since they can do EACH stat, you can get 186 points total. Basically, EVs can give you a larger boost in two single stats, but IVs can give you a higher TOTAL boost.