<p>I really think this tag needs to be edited. Dispersion, underdispersion, and overdispersion should be synonyms.</p>

<p>The dispersion parameter $\phi$ was coined by Wedderburn in 79 in his paper on quasilikelihood and this has been a term that has continued to be used in the majority of literature and software. It refers specifically to parametric estimation and certain classes of models. Overdispersion is simply $\phi &gt; 1$ and conversely for underdispersion.</p>

<p>A separate tag called "spread" could be created for robust analogues of "variance" including median absolute deviation, or trimmed mean squared error, or the like. These should not be conflated. Despite that, I concede the current wikipedia page on "statistical dispersion" mostly addresses what I am calling "spread" here. Yet, it does not cite the Wedderburn paper, nor any relevant literature on dispersion, and the cited literature mostly refers to "spread" rather than "dispersion" for the concept of robust/alternative measures of variance.</p>

<p>I agree that questions addressing the epidemiologic concept should simply be tagged "epidemiology".</p>
