Observed signal levels:

 none         DUT84
115.3819     115.1335
115.3744     115.1284
115.3722     115.1269
115.3680


      averages 
115.3741     115.1296

Standard deviations of individual measurements
 0.0058      0.0035

Standard deviations of averages
 0.0029     0.0020

Standard deviation on insertion loss 0.0035

IL = 115.3741 -  115.1296 = 0.2445 dB

Error limits (3 sigma) +/- 0.0105 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.