Listed below are brain volumes (cm3) of twins from Data Set 8 “IQ and Brain Size” in Appendix B. Construct a 99% confidence interval estimate of the mean of the differences between brain volumes for the first-born and the second-born twins. What does the confidence interval suggest?

Use the listed paired sample data, and assume that the samples are simple random samples and that the differences have a distribution that is approximately normal.

Data Set 8: IQ and Brain Size 

Data are from 20 monozygotic (identical) twins (first five rows shown here). PAIR identifies the set of twins, SEX is the gender of the subject (1 = male, 2 = female), ORDER is the birth order, IQ is measured full IQ score, VOL is total brain volume (cm3), AREA is total brain surface area (cm2), CCSA is corpus callosum (fissure connecting left and right cerebral hemispheres) surface area (cm2), CIRC is head circumference (cm), and WT is body weight (kg).
Data provided by M. J. Tramo, W. C. Loftus, T. A. Stukel, J. B. Weaver, M. S. Gazziniga. See “Brain Size, Head Size, and IQ in Monozygotic Twins,” Neurology, Vol. 50.