I'm up into my neck in a spreadsheet model of mobile broadband adoption & device usage at the moment.
An interesting paradox is currently bending my mind:
- some mobile broadband subscriptions will be "double-counted". One subscription might cover multiple devices, particularly for non-SIM technologies like Mobile WiMAX. So one individual user might have a single WiMAX "account", but have 2 or 3 separate devices permitted to use it (eg laptop + personal media player + in-car system) . A similar scenario might be people who SIM-swap between multiple devices, or share a 3G USB modem or mobile-backhauled local router.
- some other mobile broadband subscriptions will be "half-counted". One device might have access to multiple network accounts or technologies. So a laptop might have embedded WiMAX, plus an HSPA modem. Or a single operator may operate multiple network technologies, and use 802.21 in the connection software, to attach the device to whichever is available at a given location.
The easy option is to design the model to ignore these effects, and assume they're just minor or will cancel each other out. But the current roughly 30% mismatch between cellular user & subscriber data suggests that's an oversimplification too far.
Edit: The more I think about this, the more complex it gets. One of the other issues is going to be the possibility to get "ad hoc" access to WiMAX, and maybe even HSPA/LTE in the future, in the same sort of way that you can get one-off access to WiFi hotspots. If I go on a trip to 5 countries with a mobile-enabled laptop, and sign up for 2 days' WiMAX with local providers in each place - or buy a local prepay data SIM for HSPA - does that count as 5 "subscriptions"?
One thing's for sure... we're all going to have to be very wary of reported statistics on mobile broadband. I've just realised I've already got 4 subs myself - a 3 HSDPA dongle, a 3 prepay SIM in a smartphone, a new HSDPA featurephone with flatrate data on O2, and a T-Mobile 3G SIM which is actually in a 2G device.
Edit 2: No, I've got 5 subs. I forgot I'd been given another temporary week-long HSDPA modem & SIM in February by the GSMA in Barcelona. Although that one may no longer be counted as "active" depending on Telefonica's definition of churn.... See what I mean about the stats?