Said Moore: 'Oh goodness, oh my,
This demand curve is sadly awry.'
'Not so', said Working,
'A fallacy's lurking.
It isn't demand. It's supply.'
This is a reference to the Identification Problem in econometrics; when observing the data on sales of a particular good, it is difficult to determine to what extent any changes are due to a shift in demand, or supply, unless one of them is fixed and the changes are a function only of the other. This problem was researched by Elmer Working (1900 - 1968) who, along with his brother Holbrook, was a pioneer of the field of econometrics. The Working brothers were preceded by an earlier pioneer in economic statistics, Henry Ludwell Moore (1869 -1958).
Thank you, Professor Treble, for this historical limerick-econometric contribution!