diff --git a/doc/distributions/chi_squared_examples.qbk b/doc/distributions/chi_squared_examples.qbk index 7e5778b0e..821d69d5f 100644 --- a/doc/distributions/chi_squared_examples.qbk +++ b/doc/distributions/chi_squared_examples.qbk @@ -105,6 +105,67 @@ _____________________________________________ So at the 95% confidence level we conclude that the standard deviation is between 0.00551 and 0.00729. +[h4 Confidence intervals as a function of the number of observations] + +Similarly, we can also list the confidence intervals for the standard deviation +for the common confidence levels 95%, for increasing numbers of observations. + +(The standard deviation is here assumed unity, +so we can simply multiply a particular standard deviation, +0.0062789 in the example above, by these values to get the confidence limits). + +[pre''' +____________________________________________________ +Confidence level (two-sided) = 0.0500000 +Standard Deviation = 1.0000000 +________________________________________ +Observations Lower Upper + Limit Limit +________________________________________ + 2 0.4461 31.9102 + 3 0.5207 6.2847 + 4 0.5665 3.7285 + 5 0.5991 2.8736 + 6 0.6242 2.4526 + 7 0.6444 2.2021 + 8 0.6612 2.0353 + 9 0.6755 1.9158 + 10 0.6878 1.8256 + 15 0.7321 1.5771 + 20 0.7605 1.4606 + 30 0.7964 1.3443 + 40 0.8192 1.2840 + 50 0.8353 1.2461 + 60 0.8476 1.2197 + 100 0.8780 1.1617 + 120 0.8875 1.1454 + 1000 0.9580 1.0459 + 10000 0.9863 1.0141 + 50000 0.9938 1.0062 + 100000 0.9956 1.0044 + 1000000 0.9986 1.0014 +'''] + +With just 2 observations the limits are from *0.445* up to to *31.9*, +so the standard deviation might be about *half* +the observed value up to *30 times* the observed value! + +Estimating a standard deviation with just a handful of values leaves a very great uncertainty, +especially the upper limit. +Note especially how far the upper limit is skewed from the most likely standard deviation. + +Even for 10 observations, normally considered a reasonable number, +the range is still from 0.69 to 1.8, about a range of 0.7 to 2, +and is still highly skewed with an upper limit *twice* the median. + +When we have 1000 observations, the estimate of the standard deviation is starting to look convincing, +with a range from 0.95 to 1.05 - now near symmetrical, but still about + or - 5%. + +Only when we have 10000 or more repeated observations can we start to be reasonably confident +(provided we are sure that other factors like drift are not creeping in). + +For 10000 observations, the interval is 0.99 to 1.1 - finally a really convincing + or -1% confidence. + [endsect][/section:chi_sq_intervals Confidence Intervals on the Standard Deviation] [section:chi_sq_test Chi-Square Test for the Standard Deviation]