By the numbers: Apple iPhone, iPad and Mac quarterly sales

Looking at averages for the past 4 quarters smooths out seasonal spikes and gives a better view of overall trends

Apple iPad sales have been somewhat flat the past couple of years, a trend that our own Gregg Keizer reported on recently. But how flat is demand? And is the number of iPads sold actually starting to trend down?

Sales of iPads -- as well as iPhones and Macs, actually -- are cyclical, with many more sold close to Christmas than are in mid-spring. That makes it hard to look at raw quarterly numbers and see a trend. But, as blogger Dr. Drang posted with accompanying graphics, if you calculate a 4-quarter moving average (use the average of the most recent 4 quarters as each data point instead of the stand-alone quarterly number), you'll get a much more useful picture of what sales are doing.

Using data from Bare Figures, here's what moving averages look like for iPad, iPhone and Mac unit sales. While overall revenue lets you know how important various product lines are for a company, unit sales can be a better measure of waxing and waning popularity.

On the graph below, you can click (or tap on mobile) on device names in the legend to turn them on and off -- you'll need to turn off iPad and iPhone data in order to see the trend for Macs, since unit sales are so much lower. Mouse over (or tap on mobile) points to see underlying data.

Computerworld calculated moving averages; data from Bare Figures.

If you really want to dive into analysis of Apple device sales, Duke University associate professor of sociology Kieran Healy ran a time-series analysis using R's stl function to decompose the revenue data, finding out how much is seasonal, how much is a real trend and what's left over.

Executive summary of Healy's analysis: Mac sales, though small, are growing. They are also somewhat less seasonal, with a back-to-school uptick as well as holiday sales. iPad sales by revenue are declining somewhat, and "It also looks like the iPad is a Christmas present," Healy said. The iPhone "seasonal swing is declining a bit, though it’s still very large."

Healy's detailed post is here, and the R code is available on GitHub.

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