Exercise: Head circumference, sex, and height, answer 11

Question 11: How could we check the assumption of no interaction between the effects of sex and height?

Suggested answer

We could add an interaction term. We multiply sex by height and add this to the equation:

Predictor Coefficient P 95% CI
intercept 284 0.007 78 to 490
height 0.161 0.009 0.040 to 0.283
sex 88.6 0.2 -54.4 to 231.7
height times sex -0.047 0.3 -0.130 to 0.035

Predictor	Coefficient	P	95% CI
intercept	284	0.007	78 to 490
height	0.161	0.009	0.040 to 0.283
sex	88.6	0.2	-54.4 to 231.7
height times sex	-0.047	0.3	-0.130 to 0.035

The P value for the height times sex interaction is 0.3, so there is no evidence that there is an interaction and we can drop this term.

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Last updated: 20 March, 2006.