Try this for an exercise, it should take no more than 5 minutes.Open up Excel. Use Tools-Data Analysis-Random Number Generation to generate, say, 100 rows by 6 columns of random data, use any distribution you like, for example uniform. Calculate the mean of each row and plot a histogram of the means. You will always get a normal distribution if the process distribution is stable, and if the subgroup size is sufficiently large. Six will usually be enough.A common problem is when you get a mixture of two, or more, normal distributions with very different means (a bimodal distribution). For example, a repair job that takes an average of 15 minutes unless a new part is needed, in which case the average time is an hour. In that case control charts don’t work unless you can separate out the distributions.
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