The charts used are extracted from the County Tax Data Base. They show most, though not all, real estate sales that have occurred in Sun City.
The first thing done is to establish the base price used by Del Webb in the original sales. This is done by eliminating the expensive and inexpensive units and those that were well out of time frame of the model. We then use the remaining homes to estimate the base price as a function of time...This allows for the fact that Del Webb's prices increased steadily over the years for each model.
We then separate those units that were more than 10% above this base price at the time they were sold. These residences are referred to as the premium units.
We then use the re-sales of these residences to project an average price for the normal and premium units. The curve is derived by a statistical technique...a least square fit to a polynomial of order 2, 3 or 4. Models with few sales are not plotted. We simply show the raw sales.
Here is an annotated example...even includes a couple of the common errors...