Observed signal levels:

 none         DUT831
115.2985     115.0439
115.3026     115.0462
115.3038     115.0482
115.3035    


      averages 
115.3021     115.0461

Standard deviations of individual measurements
 0.0025      0.0022

Standard deviations of averages
 0.0013     0.0013

Standard deviation on insertion loss 0.0018

IL = 115.3021 -  115.0461 = 0.2560 dB

Error limits (3 sigma) +/- 0.0054 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.