Is variation really random?
One of the main assumptions of statistics is that variation follows the familiar Bell (normal) curve. While large excursions from normality are often permissible, and while methods can be developed for any distribution, standard methods can break down. To take a simple example, breast cancer is probably not normally distributed according to height. This is because breast cancer is rare in men, who also tend to be taller. So, in a graph of incidence of breast cancer according to height, the curve is likely to be skewed, with many more cases occurring among shorter people. If the results for men and women were graphed separately, however, one would expect both curves to be symmetric.
