Mac market share has been a hot topic ever since Apple staggered into (and subsequently crawled out of) the gutter in the mid-90s. It has been a slow but steady climb to the point where, as of late, Apple has set quarter after quarter of records when it comes to both Mac and iPod sales.

But, as even the casual Apple-watcher knows, there are many different metrics for measuring Mac market share these days, and they often don't match up. Browser stats, sales figures, notebooks versus desktops—it can all get a little muddy after a while. Here, we try to analyze these different statistics to get a bigger picture of what's really going on with the long-time underdog of the PC world.

Browser statistics

Some of the most commonly touted market share stats are browser statistics, specifically from Net Applications. This is because Net Applications releases its data on a monthly basis (unlike many research firms, which track different aspects of market share), so it's easy for those who are interested to follow every granular change as each month passes. Most recently, Net Applications released its January 2009 market share data, which showed Mac OS X holding 9.93 percent of the overall market (iPhone OS grabbed 0.48 percent, more than half the share of all of "Linux" combined, apparently).

Data source: Net Applications

Why they're not entirely accurate: The discrepancy in Linux numbers makes it easier to explain why Net Applications' data isn't necessarily a precise picture of the market. The company tracks OS and browser use among "member sites" that use Net Applications' tracking services, which the company says encompasses data from some 160 million users per month. This means that the only OS and browser numbers being tracked are those from users who specifically visit those member sites, which include the New York Times, the Wall Street Journal, Forbes, and InformationWeek. If specific demographics of users—like, say, Linux users—don't tend to read those types of sites, they are going to be underrepresented, and similarly, other demographics may be overrepresented.

Why they're good: That being said, browser metrics such as these aren't worthless. Even though they may be an inaccurate way to make comparisons between operating systems, they provide a good picture when it comes to trends within a specific OS. For example, Net Applications tracked the Mac OS at 7.3 percent at the end of 2007 and 9.63 percent at the end of 2008, showing more than a 2.6 percentage point jump in only a year for the Mac. In this sense, it doesn't matter if Mac users tend to visit the Wall Street Journal's website more than Linux users. The trend is clearly showing that Mac users, with all their unique browsing habits, are growing steadily.

Retail/vendor shipments

Retail shipments from various PC makers are probably the second most common market share statistics that you'll encounter online. These tend to come out on a quarterly basis. Recently, market research firm Gartner reported that Apple had grabbed roughly 8 percent of the market for the fourth calendar quarter of 2008, which was down from 9.5 percent in the previous quarter. The trend looks disturbing, and it could be if it keeps up in the long run. But, as usual, there's an explanation that makes the numbers not quite as scary as they seem.

(Preliminary sales estimates for the first calendar quarter of 2009 show a slight decrease in retail shipments compared to last quarter. Piper Jaffray analyst Gene Munster says that this trend shows that Apple is being impacted a bit by the current recession, but that the company is otherwise weathering it well by not slowing down too much.)

Data source: Gartner; IDC

Why they're not entirely accurate: There are two hidden caveats to numbers provided by firms like Gartner and IDC. For one, they focus on US shipments only, meaning that global shipments are not taken into account whatsoever in the above chart (as opposed to Net Applications' browser trends, which obviously include all traffic to the member sites no matter where the traffic is from).

Data source: IDC

Second, these numbers only focus on machines that are shipped directly to customers and to vendors/retail stores that sell to customers, including big box retailers like Best Buy, mom and pop shops down the street, and everything in between. While Best Buy may have placed a huge order for the latest iMac and inflated the numbers a bit, it doesn't necessarily mean that every one of those machines is ending up in a loving home.

This applies to both Macs and other PCs, so the fact that Acer and its sexy netbooks seem to have passed Apple last quarter in retail shipments may not actually mean that more people are buying PC netbooks, but rather that more retailers are trying to sell them to customers. It's a small but important difference.

On top of that, different firms track shipments to different retailers. IDC and Gartner often offer similar—but not exactly the same—numbers, and this is why. Neither of them are totally comprehensive.

Why they're good: Generally speaking, retailers do their best to make sure they don't have too much leftover stock. After all, it's not in their better interests to have lots of old computers sitting in the back by the time new models come out. In this sense, it's somewhat safe to assume that most retail shipments are ending up in customers' hands.

What's it all mean?

"Over the last several years [Apple] has captured a lot of mindshare, and now it's directly translating into market share," Michael Gartenberg, formerly Research Director at Jupiter Research and now VP of Strategy and Analysis at Interpret, LLC, explained to Ars. He said that both browser numbers and sales figures are important to consider, but for different reasons.

"Browser share numbers show what people are doing with these machines and how they're using them," Gartenberg said. "You often find that, if you look at other usage numbers, Apple's customers are more sophisticated in using their computers in digital media than Windows users. That goes directly to Apple's marketing message: Apple takes the time to market to users that their stuff can be used with pictures and video, which Microsoft has ignored until fairly recently. When you look at browsing statistics, you tend to see a bit of disproportional share of the market, but it's indicative of how Apple users are using their products."

Because of this, sales figures trump browser stats at the end of the day, Gartenberg noted, though no one should be upset that Apple managed to move down a spot last quarter thanks to Acer's soaring netbook sales. "Looking at Apple's numbers, it was actually Apple's best quarter, which is pretty telling, especially in a recession," he said. "The fact that we saw Acer eclipsing Apple means that people [are] buying netbooks probably as a second or third PC and they're looking for the most rock bottom price they can get." For many other consumers, however, Apple continues to do well as a primary computer. "For folks who are looking for the full PC experience, the no compromise device, Apple continues to do really well and grow in share."

Apple is most certainly on top of its game, but the challenge will be to stay on top. "Apple can't afford to take a moment to rest and breathe," Gartenberg said. "If Apple were to stop innovating, if the products became 'me too' products—which is what happened in the mid 90s—it would mean bad things." But Apple doesn't do that anymore. Instead, it chooses to ignore the mounds of advice heaped upon it by the industry, analysts, and the general public and innovates on its own schedule with its own ideas, which has worked out quite well for the company in recent years.

"The company is executing well in an economic downturn," Gartenberg said. "All it has to do is keep up the momentum and it will continue to succeed."