The usage share of operating systems is the percentage market share of the operating systems used in computers. Different categories of computers use a wide variety of operating systems (OS), so the total usage share varies significantly from one category to another. Android has the largest installed base of all general-purpose operating systems, and it is the most popular OS in Asia and Africa, by web usage. Since 2013, devices running it have been selling more than Windows, iOS and Mac OS X devices combined. That makes Android the most popular smartphone operating system, while on tablets, the iPad tablet still makes iOS more popular in that category. In certain categories, one family of operating systems dominates, for example, most desktop and laptop computers use Microsoft Windows, while most supercomputers use Linux. In the servers category, there is more diversity. Data about operating system share is difficult to obtain, since in most categories there are few reliable primary sources or agreed methodologies for its collection.