Ahodges/doc/doc1117

@mulby , @shnayder , @WatsonEmily , @mhoeber , @catong , @srpearce , @stroilova , @dsjen please review. I do not yet have images to illustrate my examples, but otherwise ready.
I wrote it up, then took it out @dsjen . I realized I was simply describing what a median was, rather than adding anything particular or interesting about our data. Happy to add it back, but prefer to include info that is unique and/or not obvious about our data and how we process it