Observed signal levels:

 none         DUT831
115.2970     115.0456
115.2947     115.0446
115.2972     115.0448
115.2965    


      averages 
115.2964     115.0450

Standard deviations of individual measurements
 0.0011      0.0005

Standard deviations of averages
 0.0005     0.0003

Standard deviation on insertion loss 0.0007

IL = 115.2964 -  115.0450 = 0.2513 dB

Error limits (3 sigma) +/- 0.0021 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.