Accuracy, Precision and Uncertainty

Counting is the only type of measurement which is free of uncertainties, as long as the number of objects or elements being counted does not change during the counting process. The result of this type of counting measurement is an example of an exact number. When we count eggs in a box, we know the exact number of eggs present in the box. Also, the number of the defined quantities are exact. From definition, 1 foot is exactly equal to 12 inches, 1 inch is exactly equal to 2.54 centimeters, and 1 gram is exactly equal to 0.001 kilograms. However, measurements other than counting are uncertain to varying degrees due to practical limitations of the measurement method used.

Accuracy, Precision & Uncertainty

Accuracy

Accuracy is the agreement between a measurement value and true value. When a clock strikes twelve and the sun is directly overhead, then the clock is considered accurate. The measurement of clock and the phenomena which is meant to measure (the sun is located or present at the zenith) are in agreement. Accuracy cannot meaningfully be discussed unless the correct value is not known or cannot be known.

Accuracy Definition

The ability of a device to measure an exact or accurate value is called accuracy. In other words, accuracy is the closeness of measured value to a standard value or true value. Accuracy is achieved with small readings. The small reading decreases the error in the calculation.

Precision

Precision is the repeatability of the measurement. It is not necessary for us to know the correct value or true value. When a clock reads exactly 10:17 a.m. when the sun is at the zenith, this clock is said to be very precise.

The proximity or closeness of two or more measurements to each other is referred to as the precision of a material or substance. If we weigh a given substance 5 (five) times and get 3.2 kg each time, our measurement is very precise, but not necessary that it is accurate. Precision is independent from accuracy.

Uncertainty

The uncertainty of a measured value is an interval around that value, so each repetition of the measurement produces a new result which falls within that interval. The experimenter allocates this uncertainty interval according to established principles of uncertainty estimation.

Uncertainty instead of error is the important term for the working scientist. Uncertainty miraculously enables the scientist to form completely certain statements.

Uncertainty in Measurement

All scientific measurements include some degree of error or uncertainty. Precision and accuracy are two essential factors related to uncertainty. Precision means how well each measurement agrees with each other, and the accuracy means how well the experimental measurement agrees with true or correct values.

Accuracy and Precision

Each measurement of the experiment is slightly different from the other and the errors and uncertainties found in it depend on the efficiency of the measuring device and the person who makes the measurement. Accuracy denotes the value that comes closest to the actual (true) value, that is it defines the difference between average value of experiment and the actual value. While precision refers to the closeness of the values ​​obtained through the measurement.

Difference Between Accuracy and Precision

There are many differences between accuracy and precision some of which are given here.

1. The degree of agreement between the actual measurement and absolute measurement is termed as accuracy. But the degree (level) of variation found in the values ​​of multiple measurements of the same factor is termed as precision. 
2. The accuracy indicates how close the measurement is to the actual measurement. But the precision determined how close a single measurement is to others.
3. Accuracy is based on only one factor, but precision is based on more than one factor. 
4. Precision focuses on systematic errors, errors caused by the problem in the appliance. In contrast, precision refers to random errors that occur periodically with no discernible pattern.

Conclusion

The ability of a device to measure an exact or accurate value is called accuracy.

The proximity or closeness of two or more measurements to each other is referred to as the precision of a material or substance.

The uncertainty of a measured value is an interval around that value, so each repetition of the measurement produces a new result which falls within that interval.

The accuracy indicates how close the measurement is to the actual measurement. But the precision determined how close a single measurement is to others.

Accuracy is based only one factor, but the precision is based on more than one factors