Well, this is definitely news. According to OsViews:

MacDailyNews has an editorial which summarizes reports from various research groups that analyzed the number of computer users affected by viruses. The conclusion was that 16 percent of all computer users are not affected by viruses because they use Macs. The lack of viruses on a Mac is commonly known, but the interesting thing is the fact that the results finally provide the first set of conclusive numbers which illustrate the Macintosh’s install-base (emphasis added).

So far only “market-share” statistics are commonly published for the public and do not convey install base. (If for example 2 people are using computers and one replaces his 2x in a 3 year period and the other only does once, market-share dynamics dictate that one demographic has 75% market share while the other has only 25% — even though install base is still 50/50.) Many tech journalists incorrectly use Apple’s 4-5% market share demographic to depict Apple’s (now known) 16% install-base.