gistfile1.txt

- (NSDecimalNumber *) standardDeviationWithPreviousPoints:(NSArray *) dataPoints
                                                    event:(CountEvent *) event {  
  NSDecimalNumber *result;
  if ([self canComputeNonMeanStatistics:dataPoints]) {
    
    
    NSDecimalNumber *accumulator = [NSDecimalNumber zero];
    NSArray *allMeans = [self arrayWithDataPoints:dataPoints event:event];
    NSDecimalNumber *mean = [self mean:allMeans];
    NSDecimalNumber *variance;
    
    for (NSDecimalNumber *number in allMeans) {
      variance = [self varianceWithActual:number mean:mean];
      accumulator = [accumulator decimalNumberByAdding:variance];
    }
    
    // we need n - 1, but thankfully, that is what we have here
    // because we added the current variance before iterating over
    // the points
    NSDecimalNumber *n = [LjsDecimalAide dnWithInt:[self dataPointsCount:dataPoints]];
    NSDecimalNumber *sigmaSquared = [accumulator decimalNumberByDividingBy:n];
    result = [self squareRootWithDecimal:sigmaSquared];
  } else {
    result = self.uncomputableStatistic;
  }
  return result;
}