diff --git a/entries/bus factor.txt b/entries/bus factor.txt
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+bus factor
+
+A number defining how many software project team members need to be run
+over by a bus before the project is no longer viable.  If you're running
+an free software or open source project and your bus factor gets too low
+then it may be time to try to recruit more developers.  Also see
+"inverse bus factor".
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diff --git a/entries/inverse bus factor.txt b/entries/inverse bus factor.txt
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+inverse bus factor
+
+The opposite of the bus factor for a software project.  This is a number
+which determines how many developers need to be metaphorically run over
+by a bus (i.e. leave the project or stop making commits) before it
+can begin to make progress again.