Define and differentiate between accuracy and precision

Accuracy is the closeness of a measurement to the true value. Precision is the closeness of agreement among a set of results.

Suppose you read a refrigerator thermometer five times and get the Celsius readings 39.0, 39.3, 39.0, 39.1, and 39.0.

The average temperature is 39.1 °C. Your readings are precise because most of the readings are within 0.1 °C of the average.

However, if the actual refrigerator is 37.0 °C, your readings are not accurate because they are off by about 2°C.

If you use a thermocouple and get an average reading of 37.00, 37.03, 37.00, 37.01, and 37.00, your readings are both accurate and more precise than those from the refrigerator thermometer.