Strain testing: THC % or Resin %?

I see strain strength based on THC % and I always wonder:

1. As a percent of what?

2. How is the THC weight of the sample determined?

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Do the seedbanks/testers do a honey oil extraction and compare the resin weight against the dry/wet weight of the bud? (not THC %)

Is the actual mass of the THC in the resin weighed? (I'm being facetious)

Does anyone know how the testing is done?

thanks

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they are all estimates, most of these seed companies do not have the resources to the kind of scientific instruments and chemical it takes to do this, besides it doesnt matter what thc percentage they have if you dont grow it correctly...