![Figure1.Scatterplot of CHD by AGE for 100 subjects](https://i.stack.imgur.com/Fdm5g.jpg)

It is written in the book [Applied Logistic Regression, Second Edition. By David W. Hosmer and Stanley Lemeshow , p.2](http://onlinelibrary.wiley.com/book/10.1002/0471722146) that a problem with the above graph is that the `variability in CHD at all ages is large`.

But I cannot visualize the variability in CHD.

Did they mean the distance between the two straight line by variability? But 0 and 1 for CHD indicate absence and presence of CHD and these two levels are just nominally defined. If I want to represent it by 1 and 1.01, then the distance will not be so large.

From what angle should I visualize the variability in CHD?