# Exercise: Head circumference, sex, and height

In a set of measurements made by a group of biomedical science, medical, nursing, physiotherapy, and radiography students, male students had a significantly bigger mean head circumference than female students. Head circumference and height were related. Could it be that the difference in head circumference between the sexes is explained by the difference in height?

The following graph shows a plot of head circumference against height for males and females:

## Questions

1. What method could we use to answer the question: is the difference in head circumference between the sexes explained by the difference in height?

The regression equation is

Head (mm) = 396 + 0.0957 × height (mm) + 6.80 × sex

where sex = 1 for a female, 2 for a male.

2. What does 6.80 tell us?

We can add confidence intervals and P values for the coefficients in this equation:

Head (mm)   =   396     +     0.0957 × height (mm)     +     6.80 × sex
95% CI                         334 to 459   0.0555 to 0.1360                   -1.1 to 14.6
P<0.001             P<0.001                           P=0.09

3. What can we conclude from this analysis?

To IAPT index.