Does Usability Follow a Normal Distribution?
Almost all statistical analyses assume that data follows a normal distribution (the famous bell curve). Most people take this on faith, because it’s true for so many phenomena. But let’s check. One way of assessing a dataset’s distribution is to draw a quantile-quantile scatterplot. In a QQ plot, we plot each observation’s empirical value on the x-axis and its hypothetical value on the y-axis, under the assumption that the entire set is normally distributed. We draw a straight line to represent a case with identical empirical and hypothetical values. If our plotted datapoints are very close to the straight line, we conclude that the empirical values are very close to the hypothetical values. In other words, the observed data are the same as what the theory predicted, so the dataset follows a normal distribution. Any datapoints that are far from the straight line represent cases in which the real and theoretical worlds differ substantially — in other words, the data doesn’t follow the
