Symmetric confidence limits
() about the estimated mean, which cover the population mean with probability. The quantity is calculated by the formula:
Here , is the critical value from the - (or Student) distribution function corresponding to the confidence level
and degrees of freedom. The symbol represents the percentile (or percentage point) of the -distribution. For 1-sided intervals,
; for 2-sided intervals,
. In each case, the confidence level is
. The confidence interval is given as
.

Note:

If the population standard deviation is known, confidence limits about a single result may be calculated with the formula:
The coefficient, is the limiting value of the -distribution function for
at confidence level.
This is identical to
, the th percentage point of the standard normal variate.

Source:

PAC, 1994, *66*, 595*
(Nomenclature for the presentation of results of chemical analysis (IUPAC Recommendations
1994))
* on page 601