Exercise: Multiple regression of muscle strength on age and height, 4

Question 4: Create a new variable to represent an interaction between age and height in their effects on strength. Add the interaction term to the regression.

Suggested answer

We use Transform, Compute, to create a new variable htage = height * age.

Click Analyze, Regression, Linear. Select Quadriceps strength into Dependent and Height, Age, and htage into Independents(s). Click OK.

The fourth table of output gives the results:

Model Unstandardized
t Sig. 95% Confidence Interval for B
B Std. Error Beta Lower Bound Upper Bound

(Constant)   4660.880 1781.580     2.616   .013 1051.056 8270.704
Height (cm) -24.736 10.439 -1.440 -2.370 .023 -45.888 -3.585
Age (years) -112.751 37.067 -11.385 -3.042 .004 -187.857 -37.645
htage .647 .219 10.617 2.961 .005 .204 1.090
a. Dependent Variable: Quadriceps strength (newtons)

The interaction term is highly significant, P = 0.005, so there is strong evidence for an interaction.

The (edited) residual plots look much better:

The distribution is less skew and the plot of residual against predicted does not appear to show a relationship.

This regression model appears to be a much better fit to the data.

Back to Exercise: Multiple regression of muscle strength on age and height.

To Applied Biostatistics index.

To Martin Bland's M.Sc. index.

To Martin Bland's home page.

This page maintained by Martin Bland.
Last updated: 19 December, 2006.

Back to top.