Three graphs, a scatter diagram, a histogram with superimposed Normal distribution curve, and a Normal plot.

The scatter diagram has the categorical variable Group on the horizontal axis with two categories: "Crohn's" and "Coeliac". The vertical axis is labeled "Skinfold (mm)" and numbered 0, 2, 4, 6, 8, and 10.

There are two columns of points. The Crohn's column goes from skinfold = 2 to skinfold = 10 and the Coeliac column from skinfold = 1.8 to skinfold = 7. Points are closer together near the bottoms of the columns than near the tops.

The histogram is for Skinfold minus mean, that is, the residuals within groups. The histogram shows positive skewness, though the small numbers makes it hard to be sure. The Normal distribution curve extends below the smallest residual.

The Normal plot has skinfold minus mean on the vertical axis and Inverse Normal on the horizontal axis. The line of points curves upwards. The points are above the straight line of perfect Normal distribution for low skinfold minus mean. The line of points goes below the straight line in the middle of the range of skinfold minus mean, and rises above the straight line for high skinfold minus mean.

Back to Clinical Biostatistics: Transformations.

This page maintained by Martin Bland.

Last updated: 3 August, 2006.