A few years ago I answered a question on math.SE about the distribution of the sample range from an exponential (1) distribution. In my answer I claim that the range and the minimum of the sample are independent, thanks to the memoryless property of the exponential distribution. Here’s a direct derivation of the independence of the range and minimum, for those (including the graduate student who contacted me about this last month!) for whom the memoryless justification is not sufficient.

Let be the order statistics from a sample of size *n* from an exponential (1) distribution. The joint distribution of and is given by

Now, let’s do a double variable transformation. Let ; i.e., *R* is the sample range. Let , the minimum. Then and . The Jacobian of the transformation is . Substituting, we have

Since factors into a function of *r* times a function of *s*, *R* and *S* are independent. (We have ; and .)