Add negative signifiance convention
@@ -111,11 +111,23 @@ significance" using the following recipe:
p_value = chi2.sf(ts, df=df)
return norm.isf(0.5 * p_value)
-In particular, with only one degree of freedom (e.g. flux amplitude), one can
-estimate the (2-sided) statistical significance in terms of number of :math:`\sigma`
-as :math:`\sqrt{TS}`.
+In particular, with only one degree of freedom (e.g. flux amplitude), one
+can estimate the statistical significance in terms of number of :math:`\sigma`
+as :math:`\sqrt{\Delta TS}`.
+In case the excess is negative, which can happen if the background is overestimated
+the following convention is used:
+
+.. math::
+
+ \sqrt{\Delta TS} = \left \{
+ \begin{array}{ll}
+ -\sqrt{\Delta TS} & : \text{if} \text{Excess} < 0 \\
+ \sqrt{\Delta TS} & : \text{else}
+ \end{array}
+ \right.
+
Counts statistics classes
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