would be the one off by 20 degree.
The chances of 3 different thermometers being off by the same amount is remote, and you could do statistics, F-tests and t-tests depending on the exact experiment to give a probability that the three measurements were randomly the same.
The error you describe occurs when the same measuring device, or the same measurement error occurs on all measurements. for instance, if you measure with a thermometer that has not been calibrated, and is consistently off by 20 degrees. By using multiple thermometers and multiple labs, you eliminate this particular error.
Radiometric dating is reasonably accurate and credible.
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