hetero-skeda-sticity, another example

[[Prem Mann]] has an example of explaining food expenditure using household income. The homo-skeda-sticity assumption on P592 is something like

   "the dispersion among expenditure of low-income households is, say, 21.1. The dispersion among high-income households is that same value. Here we mean the dispersion among the residual values i.e. the unexplained portion of expenditure."

P598 further clarifies that the POPULATION "Spread of errors" at a given income level is a different quantity than that of the SAMPLE.

Note this is an assumption about the population not a sample. Suppose 5 income levels. A small sample having just 2 households per level (10 households in entire sample) will be too small, and is very likely to show inconsistent dispersions at low-income vs high-income.

Needless to say Dispersion is measured by stdev.

This book has some nice diagrams about the dispersions at 2 income levels.