http://s32.postimg.org/u235jc65x/formula.png
The Sigma Algorithm
1). Start with a set of kills, for example {NA, Soraka, Support}
2). Calculate the total kills, totalKills = sum of all kills in the set
3). Calculate the average kills per game from the set of games played, averageKillsPerGame = totalKills / numberOfGamesInSet
From here we want some sense of how each game was different from that theoretical average, a deviation from average.
4). For each game in the set, calculate the distance between that game's kills and the set average
gameDeviation = thisGameKills - averageKillsPerGame
5). Produce only positive distance numbers by squaring the result
gameDeviationSquared = gameDeviation * gameDeviation
6). Store those gameDeviationSquared values and calculate the average of that new set of deviationsSquared
averageDeviationSquared = totalDeviationsSquared / numberOfDeviationsSquared
7). Calculate Sigma, the standard deviation for the set:
standardDeviation = squareRoot( averageDeviationSquared )
standardDeviation is a kind of average deviation away from the set average.