Phil Schiller stated (under oath) that when assessing sales for a new model of the iPhone, Apple used an easy shorthand:

Each new generation sold approximately equal to all previous generations combined.

That’s a helpful gauge. Did it hold and will it continue to hold?

In order to test this rule of thumb we need to know generational sales. Although we know overall unit sales for a given quarter, since Apple has been selling several versions simultaneously, we need to make some guesses about the ratio of generations in the mix. My guess is about 10% for 3G vs. 90% 3GS and 5% 3GS vs. 10% 4 vs. 85% 4S. That results in the following pattern:

The cumulative totals for these generations can then be generated and they appear as follows: