Observed signal levels:

 none         DUT73
115.3357     115.2500
115.3358     115.2512
115.3372     115.2532
115.3385

      averages 
115.3368     115.2515

Standard deviations of individual measurements
 0.0013      0.0016

Standard deviations of averages
 0.00065     0.00043

Standard deviation on insertion loss 0.0008

IL = 115.3368 -  115.2515 = 0.0853 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.