Why Apple is touting its active installed device base

Announcing its results for the three months to December, which marks the firm’s fiscal first quarter, Apple on Tuesday revealed a 15 percent slide in iPhone revenue compared to the year-ago period.

With device sales slowing down — though we don’t know the exact number, as the company has stopped providing unit sales figures — the US tech giant now wants investors to focus instead on another metric: installed base of the phones.

The message is this: Look at our installed base of devices and you will get an idea of…