Science Puzzle
Which Measurement Can You Trust?
Two thermometers are placed in a beaker of pure melting ice, whose true temperature is 0 degrees Celsius.
Thermometer A reads 5.01, 5.02, 5.00, 5.01. Thermometer B reads minus 1.2, plus 0.8, minus 0.5, plus 0.9. Which one would you rather rely on?
The Answer
Thermometer B, despite looking messier. This is the difference between precision and accuracy, two words we treat as synonyms in daily speech and which mean quite different things in science.
Precision is how closely repeated readings agree with each other. Accuracy is how close they are to the truth. Thermometer A is beautifully precise: four readings within a hundredth of a degree. It is also wrong by five whole degrees, every single time. That is a systematic error, a bias, and no amount of repeating will reveal it. Its confidence is an illusion.
Thermometer B scatters, so any one reading could be off by a degree. But the readings cluster around zero, the true value. Average its four readings and you get very close to the truth. Random scatter can be beaten by taking more measurements; a hidden bias cannot.
This is why scientists calibrate instruments against a known standard, and why a very precise instrument that has never been checked is one of the most dangerous things in a laboratory.
The principle: Accuracy versus precision. Precise readings agree with one another; accurate readings agree with reality. Random error can be averaged away, systematic bias cannot.