In an item posted Monday, Asymco‘s Horace Dediu invites readers to look over his shoulder as he estimates Apple’s (AAPL) iOS device sales for 2012 by three different methods.

• Top down analysis: Using a recent market-data report estimating that total smartphone and tablet sales will grow to 6.2 billion units over the next five years, Dediu assumes that Apple’s share of those sales is likely to grow from 17% in 2011 to 20% by 2016. Extrapolating on a straight-line basis, this method yields iOS sales totaling 225 million in 2012.
• Production estimates: Noting that Apple usually sells iPhones and iPads as fast as it can make them, Dediu forecasts 2012 production by assuming that Apple’s current ramp-up rates will reflect the company’s capital expenditures on new factory equipment. This method yields total 2012 sales of 260 million iOS devices.
• Customer behavior: “If we know the probability of upgrades and the historic split between ‘new’ and ‘existing’ users,” Dediu writes, “then we can roll the forecast forward.” This method, illustrated in the chart above, yields an estimate of 280 million iOS devices.

The consumer behavior technique is probably the least dependable because the new/existing user ratios on which it is based have varied so widely over the years.

But, Dediu concludes, “it’s interesting to note that the three methods yield growth rates of 46%, 69% and 82%.”  Wall Street’s forecast for Apple is that its 2012 sales will grow less than 25%, which leaves considerable room for an upside surprise.

You can read Dediu’s full report here.