Observed signal levels:

 none         DUT831
115.2954     115.0258
115.2937     115.0291
115.2941     115.0290
115.2946


      averages 
115.2945     115.0280

Standard deviations of individual measurements
 0.0007      0.0019

Standard deviations of averages
 0.0004     0.0011

Standard deviation on insertion loss 0.0011

IL = 115.2945 -  115.0280 = 0.2665 dB

Error limits (3 sigma) +/- 0.0034 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.