By Philip Elmer-DeWitt
November 10, 2013

FORTUNE — These two charts at right, taken from a YouGov survey and chosen for demonstration purposes by Charles Arthur in the Guardian last week, illustrate nicely what’s wrong with statements like this (from TheNextWeb): “Android represents somewhere in the region of 80% of the smartphone market.”

That’s “simply wrong,” Arthur writes.

“Here’s the reality: at the time this was written, more than 40% of the smartphones in use in the US… were iPhones. Only about 51% of the smartphones in peoples’ hands in the US are Android phones. The ratios are more in Android’s favour elsewhere, but nowhere outside of China (and perhaps India) would you find four in five smartphone owners using an Android phone.”

The purpose of Arthur’s Guardian piece was to explain — once and for all — the difference between market share and installed base, between units shipped and units sold, between new sales and replacement sales, between saturated markets and those that still have a long way to grow.

Daring Fireball‘s John Gruber calls Arthur’s story “the best piece I’ve ever seen on the folly of putting too much stock into ‘market share’ as a metric.”

As someone who spends more time than I probably should obsessing about Apple’s (AAPL) market share numbers, I second that.

LINK: Why an 80% market share might only represent half of smartphone users.

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