The question about milk insufficiency was hotly debated when the Wall Street
Journal published its "Dying for Milk" article. The 5% number which
circulates regarding the percentage of women unable to produce sufficient
amounts of milk is an estimate from a referred population and does not
represent the percentage in the general population. If it did, the human race
would have died out before formula comapnies rescued us all from our
inadequacies.

I use data from Michael Woodridge's study which estimates true
pathophysiological lactation failure at 1.3% This was published in Nutrition
in Child Health, edited by DP Davies, Royal College of Physicians of London
Press, Ch. 2, Breastfeeding: physiology in to Practice, 1995, pp.13-31.

Hope this is helpful