Observed signal levels:

 none         DUT318
115.2975     115.0983
115.2975     115.1006
115.2970     115.1022
115.2984


      averages 
115.2976     115.1004

Standard deviations of individual measurements
 0.0006      0.0020

Standard deviations of averages
 0.0003     0.0007

Standard deviation on insertion loss 0.0008

IL = 115.2976 -  115.1004 = 0.1972 dB

Error limits (3 sigma) +/- 0.0024 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.