Observed signal levels:

 none         DUT34
115.3274     115.2530
115.3262     115.2556
115.3286     115.2557
115.3290

      averages 
115.3278     115.2547

Standard deviations of individual measurements
 0.0013      0.0015

Standard deviations of averages
 0.00065     0.00087

Standard deviation on insertion loss 0.0011

IL = 115.3278 -  115.2547 = 0.0730 dB

Error limits (3 sigma) +/- 0.0033 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.