Sir.—The study by Mathew1 in the February 1990 issue of AJDC concludes that "differences in hole sizes primarily account for the observed variability in milk flow." This would not surprise many grandmothers.
Nevertheless, that common sense conclusion is not supported by the data presented in the article's figures. For example, for a given hole size, the observed milk-flow rates varied by a factor of three or more (in the first figure) depending on the particular type of nipple, with some variation within nipple types. This naturally raises some questions (beyond the obvious one of manufacturers' quality control):
1. What about nipple distensibility, which the author related to nipple wall thickness at the tip of the nipple? Hole sizes and wall thicknesses were apparently measured with no pressure applied to the nipple. Perhaps the wall thickness changes under pressure. The author's experiments were done at a single pressure of