Although Apple has done a better job of moving its Mac users along with each new operating system than has rival Microsoft, the Cupertino, Calif. company has been unable to eradicate fragmentation as it accelerated upgrades to an annual cadence.

According to data from analytics firm Net Applications, three OS X editions that were three years or older retained five or more percentage points of user share last month. Those three editions — 2009’s Snow Leopard, 2011’s Lion and 2012’s Mountain Lion — powered 20 percent of all Macs in April. When 2007’s Leopard was included, the number climbed to 21.3 percent.

There’s no question that Apple’s policy of giving away its OS X upgrades — a practice begun in 2013 with Mavericks — has reduced fragmentation by pulling Mac owners onto the newest edition faster than did versions that carried a price tag.

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