Observed signal levels:

 none         DUT8
115.1412     114.9751
115.1403     114.9763
115.1407     114.9775
115.1437

      averages 
115.1415     114.9763

Standard deviations of individual measurements
 0.0015      0.0012

Standard deviations of averages
 0.00075     0.00069

Standard deviation on insertion loss 0.0010

IL = 115.1415 -  114.9763 = 0.1652 dB

Error limits (3 sigma) +/- 0.0030 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.