Question

A study of iron deficiency among infants compared samples of infants following different feeding regimens. One group contained breast-fed infants, while the infants in another group were fed a standard baby formula without any iron supplements. Here are summary results on blood hemoglobin levels at 12 months of age. Give a 95% confidence interval for the mean difference in hemoglobin level between the two populations of infants. Round answers to two decimal places.

n x s

Breast-fed 23 13.1 1.6

Formula 19 12.7 1.7

Answer 1

Degrees of freedom = (s1^2/n1 + s2^2/n2)^2/((s1^2/n1)^2/(n1 - 1) + (s2^2/n2)^2/(n2 - 1))

                                = ((1.6)^2/23 + (1.7)^2/19)^2/(((1.6)^2/23)^2/22 + ((1.7)^2/19)^2/18)

                                = 37

At 95% confidence level the critical value is t^* = 2.026

The 95% confidence interval is

() +/- t^* * sqrt(s1^2/n1 + s2^2/n2)

= (13.1 - 12.7) +/- 2.026 * sqrt((1.6)^2/23 + (1.7)^2/19)

= 0.4 +/- 1.04

= -0.64, 1.44