Observed signal levels:

 none         DUT37
115.3329     115.2519
115.3320     115.2522
115.3351     115.2556
115.3356

      averages 
115.3339     115.2532

Standard deviations of individual measurements
 0.0017      0.0020

Standard deviations of averages
 0.00085     0.00115

Standard deviation on insertion loss 0.0012

IL = 115.3339 -  115.2532 = 0.0806 dB

Error limits (3 sigma) +/- 0.0036 dB

Errors are estimated from too few data points so
error estimation for this particular measurement is 
uncertain.
There are many measurements in this series so the statistics
of all the errors estimates will give a good idea about errors.