Standard Statistical Tests for normality

[this page | pdf | back links]

Standard statistical tests for identifying whether an observed sample is likely not to have come from a normal distribution include:


(a)    Testing the extent to which the skew of the sample is non-zero, see e.g. Confidence level for skew for large sample normal distribution


(b)   Testing the extent to which the (excess) kurtosis of the sample is non-zero, see e.g. Confidence level for (excess) kurtosis for large sample normal distribution


(c)    The Jarque-Bera test which simultaneously tests the extent to which the skew and (excess) kurtosis of the sample are non-zero


(d)   The Shapiro-Wilk test


(e)   The Anderson-Darling test*


(f)     The Kolmogorov-Smirnov test*


(g)    The Cramer-von-Mises test*


* These tests can be used with any distributional form, i.e. they are not limited to testing merely for non-normality. Their test statistics depend on the sample data through terms that depend merely on order statistics and then only on how these map onto the hypothesised cumulative distribution function (i.e. if the sample is  then merely through  where  where  is the cumulative distribution function and  is the ’th order statistic, i.e. the ’th smallest value in the sample). In contrast (a) to (c) are parametric, with their test statistics depending merely on specific moments of the distribution (here the skew and kurtosis and the two combined respectively). (d) depends on both order and parametric elements.


All of the above tests, as conventionally formulated, have the disadvantage that they give ‘equal’ weight to every observation. A possible exception is the Kolmogorov-Smirnov test, which merely refers to the single (ordered) observation that appears to exhibit the greatest deviation from where we might have expected it lie.


As explained in Kemp (2009), this generally means that they indicate mainly whether a sample appears to be deviating from normality in the middle of the distribution rather than whether it appears to be deviating from normality in its tails. Loosely speaking, this is because there are far more observations in the middle of a normal distribution than in its tail. We illustrate this with (b). Consider the proportion of observations that are in the tails of a normal distribution. Only approximately 1 in 1.7 million observations from a normal distribution should be further away from the (sample) mean than 5 standard deviations. Each one in isolation might on average contribute at least 625 times as much to the computation of kurtosis as an observation that is just one standard deviation away from the (sample) mean (since 5 x 5 x 5 x 5 = 625), but, because there are so few observations this far into the tail, they in aggregate have little impact on the overall kurtosis of the distribution.


Better, if we are interested merely in testing for deviation from normality in a part of a distributional form is to modify the above methodologies so that they depend just on data from the relevant part of the observed distributional form. For example, we might wish to focus on the worst 10% of outcomes. We would then estimate the mean and standard deviation of a normal distribution that would have its worst 10% of outcomes as close as possible to those actually observed, and we would then apply a modified test statistic that referred merely to the observations in the part of the distributional form in which we are interested. In general, we can view this modification as involving giving different weights  to the different . To calculate critical values for such statistics (and therefore whether or not to reject the null hypothesis of normality) generally requires Monte Carlo simulation techniques, given the wide range of possible weighting schemas that could be used.


Contents | Next

Desktop view | Switch to Mobile