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Installed base (also install base, install[ed] user base or just user base) is a measure of the number of units of a particular type of system—usually a computing platform—actually in use, as opposed to market share, which only reflects sales over a particular period. Although the install base number is often created using the number of units that have been sold within a particular period, it isn't necessarily restricted to just systems, as it can also be products in general. Because installed base includes machines that may have been in use for many years, it is usually a higher figure than market share. Many people see it as a more reliable indicator of a platform's popularity.
Installed base is not the same as the total number of units sold at any given moment in time (cumulative sales numbers), since some of those units will typically be out of use because they have broken, gone missing, been made obsolete or replaced by newer versions.