Skip to main content
Mail a PDF copy of this page to:
(Your email address will not be added to a mailing list)
Show menu

Reference interval

Next selectReference intervals
Next selectReference interval


A Reference interval (Reference range, Normal range) can be calculated using the following 3 methods: (a) using the Normal distribution (b) using a non-parametrical percentile method, and (c) optionally the robust method as described in the CLSI Guidelines C28-A3.

Required input

Input for reference interval

In the dialog box you identify the variable with the measurements. You can click the Drop-down button button to obtain a list of variables. In this list you can select a variable by clicking the variable's name. You can also enter or select a filter in order to include only a selected subgroup of measurements in the statistical procedure, as described in the Introduction part of this manual.



Reference interval

Summary statistics

Logarithmic transformation

If the option Logarithmic transformation was selected, the program will display the back-transformed results. The back-transformed mean is named the Geometric mean. The Standard deviation cannot be back-transformed meaningfully and is not reported.

Suspected outliers

The program produces a list of possible outliers, detected by the methods based on Reed et al. (1971) or Tukey (1977). The method by Reed et al. tests only the minimum and maximum observations; the Tukey test can identify more values as outliers. Note that this does not automatically exclude any values from the analysis. The observations should be further inspected by the investigator who can decide to exclude the values. Click on the listed values (which are displayed as hyperlinks) to show the corresponding data in the spreadsheet (see Exclude & Include).

Reference interval

The program will give the 90, 95, 99, 99.9 or 99.99% Reference interval, double sided or left or right sided only, as selected in the dialog box.

The reference interval is calculated using 3 different methods: (a) using the Normal distribution (Bland, 2000; CLSI 2008), (b) using a non-parametrical percentile method, and (c) optionally a "robust method" as described in the CLSI Guidelines C28-A3.

90% Confidence Intervals are given for the reference limits.

For the robust method the confidence intervals are estimated with the bootstrap method (percentile interval method, Efron & Tibshirani, 1993). When sample size is very small and/or the sample contains too many equal values, it may be impossible to calculate the CIs.

The results from the Normal distribution method are not appropriate when the Test for Normal distribution (see above) fails. If the sample size is large (120 or more) the CLSI C28-A3 guideline recommends the percentile method, and for smaller sample sizes the "robust method".

The minimal sample size of 120 for the percentile method is the minimum number required to calculate 90% Confidence Intervals for the reference limits. A higher number of cases is required to achieve more reliable reference limits with more narower 90% Confidence Intervals.


Click the Graph button in the dialog box shown above to obtain the following Reference Interval Graph box:

Reference interval graph dialog box

This results in the following graph:

Reference interval graph


See also

External links