Observed signal levels:

 none         DUT81
115.3074     115.0781
115.3082     115.0792
115.3084     115.0813
115.3079

      averages 
115.3079     115.0795

Standard deviations of individual measurements
 0.0004      0.0016

Standard deviations of averages
 0.0002     0.0009

Standard deviation on insertion loss 0.0009

IL = 115.3079 -  115.0795 = 0.2284 dB

Error limits (3 sigma) +/- 0.0027 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.