The numbers 3.92, 3.29 and 5.15 need to be replaced with slightly larger numbers specific to the t distribution, which can be obtained from tables of the t distribution with degrees of freedom equal to the group sample size minus 1. If the sample size is small (say less than 60 in each group) then confidence intervals should have been calculated using a value from a t distribution.

The standard deviation for each group is obtained by dividing the length of the confidence interval by 3.92, and then multiplying by the square root of the sample size:ДЇor 90% confidence intervals 3.92 should be replaced by 3.29, and for 99% confidence intervals it should be replaced by 5.15. If the sample size is large (say bigger than 100 in each group), the 95% confidence interval is 3.92 standard errors wide (3.92 = 2 Г— 1.96). Most confidence intervals are 95% confidence intervals. Again, the following applies to confidence intervals for mean values calculated within an intervention group and not for estimates of differences between interventions (for these, see Section 7.7.3.3). When making this transformation, standard errors must be of means calculated from within an intervention group and not standard errors of the difference in means computed between intervention groups.Д¬onfidence intervals for means can also be used to calculate standard deviations. 7.7.3.2 Obtaining standard deviations from standard errors and confidence intervals for group meansДЄ standard deviation can be obtained from the standard error of a mean by multiplying by the square root of the sample size: For the current version, please go to /handbook/current or search for this chapter here. This is an archived version of the Handbook.

How does excel calculate standard error