This rang a bell & I found this in my Commonplace Book:
If we collect a man's urine during 24 hours & mix all this urine to analyse the average, we get analysis of a urine which simply does not exist; for urine when fasting is different from urine during digestion. A startling instance of this kind was invented by a physiologist who took urine from a railroad station urinal where people of all nations passed, & who believed he could thus present an analysis of an average European urine!
Claude Bernard 1865
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Of course, in the drugs case they are using the average in a legitimate way, as a preserver of mass
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