This allows us to be more effective in predicting outcomes. A 21 year old male is much more active than an 85 year male. If we compared the strength of this 21 year old to the strength of an 85 year old male, even if there was a significant strength deficit, it would still look pretty good. By comparing him to another 21 year old we get a true representation of where his strength is and where it should be compared to other 21 year olds his age and weight. By comparing you to someone your age, weight and gender we are able to more accurately predict your response to our treatment.