You get meaningful reference values, if you might have a sample size
of at least 120 subjects (see CLSI guideline C28-4).
In SPSS, you could calculate 2.5% + 97.5% percentiles,or other
percentiles (95%), using -->analyze --> frequency --> statistics.
FREQUENCIES
VARIABLES=M4.1 /FORMAT=NOTABLE
/PERCENTILES= 95
/ORDER= ANALYSIS .
However, there are a lot of possibilities how to estimate a percentile
with SPSS (Default is waverage, ...).
Search for "Methods for computing percentiles" within Google, and look
at the MedStats-entry. Here, several options how to calculate
percentiles are discussed.
And: the CI of reference limits are not given in SPSS.
The free available paper describes calculation of CI:
Campbell MJ, Gardner MJ (1988): Calculating confidence intervals for
some non-parametric analyses. British Medical Journal, 296, 1454-1456
http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1833334&blobtype=pdf
Best regards
Thomas Keller
ACOMED statistik