How To Calculate Standard Deviation Given Mean And Sample Size

7.seven.3.ii Obtaining standard deviations from standard errors and conviction intervals for group means

A standard departure can be obtained from the standard error of a mean past multiplying by the square root of the sample size:

When making this transformation, standard errors must exist of means calculated from within an intervention group and not standard errors of the difference in ways computed betwixt intervention groups.

Confidence intervals for means can also be used to summate standard deviations. Over again, the following applies to confidence intervals for mean values calculated within an intervention group and not for estimates of differences betwixt interventions (for these, encounter Section 7.7.3.3). Nearly confidence intervals are 95% confidence intervals. If the sample size is large (say bigger than 100 in each group), the 95% conviction interval is 3.92 standard errors wide (3.92 = ii × ane.96). The standard deviation for each group is obtained past dividing the length of the conviction interval by 3.92, and then multiplying past the square root of the sample size:

For 90% confidence intervals 3.92 should be replaced past 3.29, and for 99% conviction intervals it should be replaced by 5.15.

If the sample size is small (say less than 60 in each group) and so confidence intervals should have been calculated using a value from a t distribution. The numbers 3.92, 3.29 and five.15 need to exist 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 one. Relevant details of the t distribution are available as appendices of many statistical textbooks, or using standard reckoner spreadsheet packages. For example the t value for a 95% confidence interval from a sample size of 25 can be obtained by typing =tinv(1-0.95,25-1) in a cell in a Microsoft Excel spreadsheet (the result is 2.0639). The divisor, 3.92, in the formula above would be replaced by ii × 2.0639 = 4.128.

For moderate sample sizes (say between 60 and 100 in each group), either a t distribution or a standard normal distribution may have been used. Review authors should await for evidence of which one, and might use a t distribution if in incertitude.

Every bit an example, consider information presented as follows:

Group	Sample size	Mean	95% CI
Experimental intervention	25	32.1	(30.0, 34.2)
Control intervention	22	28.3	(26.5, 30.1)

The confidence intervals should have been based on t distributions with 24 and 21 degrees of freedom respectively. The divisor for the experimental intervention group is 4.128, from above. The standard deviation for this group is √25 × (34.2 – 30.0)/4.128 = v.09. Calculations for the control grouping are performed in a like fashion.

It is of import to check that the confidence interval is symmetrical most the mean (the distance between the lower limit and the mean is the same as the distance between the mean and the upper limit). If this is not the case, the confidence interval may accept been calculated on transformed values (see Section 7.7.3.4).