Observed signal levels:

 none         DUT6
115.3158     115.2047
115.3149     115.2022
115.3143     115.2056
115.3128


      averages 
115.3145     115.2042

Standard deviations of individual measurements
 0.0013      0.0018

Standard deviations of averages
 0.0006     0.0010

Standard deviation on insertion loss 0.0012

IL = 115.3145 -  115.2042 = 0.1102 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.