Fig 1. Sample histogram. Percentile mean and SD measures are calculated from the top 50%, 25%, and 10% of the histogram curve. Skewness is zero if the data are distributed symmetrically around the mean, negative if the data are more spread out on the left of the mean, and positive if the data are more spread out on the right of the mean. Kurtosis, a measure of how "peaked" the histogram is, equals zero if the histogram is Gaussian, is positive if the histogram has a sharper peak, and is negative if it has a flatter top.