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TED BYRT, MED
Clinical Epidemiology and Biostatistics Unit Royal Children's Hospital Flemington Rd Parkville, Victoria 3052, Australia

Am J Dis Child. 1991;145(3):250-251.

	Since this article does not have an abstract, we have provided the first 150 words of the full text PDF and any section headings.

Sir.—In his article on P values¹ published in the April 1990 issue of AJDC, Brown has made some very helpful comments on the meaning of P values and has drawn attention to the difficulties associated with performing repeated statistical comparisons on the same set of subjects. However, twice in his article he does not pay sufficient heed to his own advice.

First, he correctly points out that when a t test is used to compare the birth weights of two groups of infants and the significant P value is set at .05, the random variation in birth weights among the infants would suggest, wrongly, in about one comparison of 20, that an important difference between the two groups in average weight was present. He goes on to consider what happens when a second test, eg, on head circumference, is performed on the same group, and says "The chance of . . . [Full Text PDF of this Article]

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