Observed signal levels:

 none         DUT5
115.3138     115.2474
115.3169     115.2466
115.3109     115.2470
115.3155


      averages 
115.3143     115.2470

Standard deviations of individual measurements
 0.0026      0.0004

Standard deviations of averages
 0.0013     0.0002

Standard deviation on insertion loss 0.0013

IL = 115.3143 -  115.2470 = 0.0673 dB

Error limits (3 sigma) +/- 0.0039 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.