Karina J. Wilkie, Students' use of variables and multiple representations in generalizing functional relationships prior to secondary school, Educational Studies in Mathematics, Vol. 93, No. 3 ...

Leo A. Aroian, The Probability Function of the Product of Two Normally Distributed Variables, The Annals of Mathematical Statistics, Vol. 18, No. 2 (Jun., 1947), pp. 265-271 ...

Footprint charts provide the benefit of analyzing multiple variables in a focused diagram. Common footprint charts include footprint profile, bid/ask footprint, delta footprint, and volume footprint.