In humans, live births tend to follow nine months after conception. The gestation period of an iOS device is closer to that of a whale or rhinoceros: new model Macs, iPads and iPhones tend to arrive 12 to 15 months after a bump in capital expenditures (plants, equipment, etc.).
Asymco’s Horace Dediu was the first analyst to spot the pattern. Last year he used it to predict that Apple in 2014 would ship between 250 million and 285 million iOS devices.
Actual shipments: 247 million, according to the Form 10-K that Apple filed Monday. Or 251 milllion if you count the Apple TV.
On Tuesday, Dediu used this year’s anticipated Cap Ex spending — $12.4 billion, not counting retail — to predict next year’s iOS shipments.
Result: 310 million iOS devices, give or take a few million Apple Watches.
Link: How many iOS devices will be produced in 2015?
Follow Philip Elmer-DeWitt on Twitter at @philiped. Read his Apple (AAPL) coverage at fortune.com/ped or subscribe via his RSS feed.