Comparison of mean of subpopulation with mean of entire population

How can I compare mean of subpopulation with mean of entire population? I want to compare mean of specific method with mean of all methods in a proficiency testing(PT) program.
A PT program is a program in which an organisation which executes PT, sends samples to multiple participants and ask them to measure some characteristics in sent sample. After a deadline participants send their results to PT provider. Provider analyse results in all and different groups. Groups are those who used same methods for measuring that characteristic.
With best regards
Morteza Izadyar