Observed signal levels:

 none         DUT381
115.2913     115.0402
115.2901     115.0426
115.2896     115.0445
115.2921


      averages 
115.2908     115.0424

Standard deviations of individual measurements
 0.0011      0.0022

Standard deviations of averages
 0.00055     0.00127

Standard deviation on insertion loss 0.0014

IL = 115.2908 -  115.0424 = 0.2483 dB

Error limits (3 sigma) +/- 0.0042 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.