Observed signal levels:

 none         DUT18
115.3121     115.1198
115.3103     115.1234
115.3113     115.1245
115.3109


      averages 
115.3112     115.1226

Standard deviations of individual measurements
 0.0008      0.0025

Standard deviations of averages
 0.0004     0.0014

Standard deviation on insertion loss 0.0015

IL = 115.3112 -  115.1226 = 0.1886 dB

Error limits (3 sigma) +/- 0.0044 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.