  Unwanted variation can be highly problematic and so its detection is often
crucial. Relative log expression (RLE) plots are a powerful tool for
visualising such variation in high dimensional data. We provide a detailed
examination of these plots, with the aid of examples and simulation, explaining
what they are and what they can reveal. RLE plots are particularly useful for
assessing whether a procedure aimed at removing unwanted variation, i.e. a
normalisation procedure, has been successful. These plots, while originally
devised for gene expression data from microarrays, can also be used to reveal
unwanted variation in many other kinds of high dimensional data, where such
variation can be problematic.
