Proxy Factor is working name for a factor that that is used as an approximation of another factor that is difficult to measure or not available for measuring. It's essentially a quick-and-dirty stand-in for the real Mc Coy.
For example, Lines Of Code can be a Proxy Factor for "code complexity". Personal preference of code style can be used as proxy factor for "grokkability" of the code ("I prefer reading code style X over Y").
They are imperfect replacements for the original target factor and shouldn't be regarded as a reliable replacement for the original. They may be sufficient for rough analysis. If measurements of the real factor are not practical to obtain, then one will have to work with proxy factors instead, being their are no available alternatives. Some may argue that using proxy factors is worse than doing no analysis, but I reject this perfect-or-nothing viewpoint.
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See original on c2.com 