Replying to Math project help

i have to do a math project. this is the "story" for it. somebody wrote a threat, and 3 people confessed to writing the letter, but only one did it. theres 25 documents from each kid, and they took the percentage ofof "and/by/the" in the paper, and the percentage of words writen off the line. i put all thsoe numbers into a list on my calculater (ti 86) and i have to show 2 ways who the person who wrote it is. i can use averages, standard deviation, and that stuff. how can i use standard deviation to prove who did it? i dont really understand what it is so an explanation of that might help.