IDC’s quarterly estimates of PC shipments are generally taken as gospel, but how accurate are they?
That’s hard to tell, because most vendors don’t report their own unit sales numbers.
But this go-round, for the first time, we have good benchmark.
That’s because Apple — which does tell investors how many Macs it sells each quarter — broke out of “Others” in IDC’s latest report and into the ranks of the top five vendors worldwide.
For the record:
- IDC estimated two weeks ago that Apple shipped 4.982 million Macs in calendar Q3
- Apple announced Monday that it actually sold 5.52 million Macs in that period
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In percentage terms, IDC said Mac sales would grow 8.9% year over year in an industry that was shrinking by -1.7%. As it turned out, Mac sales grew 18%.
Compared with the analysts Fortune polled, that put IDC’s estimate in the middle third — worse than 12 analysts, better than 20.
[Note that IDC estimates shipments while Apple reports sales. Because Apple ended the quarter with Mac channel inventory slightly below its usual target range, we can assume those two numbers are comparable.]
Follow Philip Elmer-DeWitt on Twitter at @philiped. Read his Apple (AAPL) coverage at fortune.com/ped or subscribe via his RSS feed.