Accuracy and Precision Measurements

Accuracy and precision are important characteristics of any measurement. To understand the difference between the two and why it is important to know which you’re dealing with, you need to understand each. Accurate measurements are close to the true value, with precision referring to how reproducible the measurements are, even if they are far from the expected value. Accuracy is the degree to which a measurement agrees with the accepted value, while precision reflects how reproducible measurements are, even if they vary greatly from the accepted value.

The uncertainty and precision are the two major purposes of calibration. In short, the process makes a measuring instrument more accurate in measuring. 

Accuracy

An instrument can be said to be accurate if it shows the exact value of a physical quantity. Accuracy generally varies according to the method used to obtain a measurement and the precision with which that method has been applied. Accuracy is the degree of agreement between a measured value and the accepted standard. The accuracy of a measuring instrument in testing depends on its precision to produce the same results each time an observation is made. In this specification, the accuracy of the system is classified into three types according to the range:

• Point Accuracy
• Accuracy as percentage of the scale range
• Accuracy as percentage of the true value

Point Accuracy

Point accuracy is achieved when the measurement of a physical quantity is taken. This means that the reading is a valid measurement only at that point on the scale. Point accuracy does not give any information about the general accuracy of the instrument.

Accuracy as a percentage of the scale range

The accuracy of a device is dependent on the scale range it possesses. The smaller the range, the higher the accuracy of measurement. For example, consider a thermometer having a scale range up to 200º C. The thermometer has an accuracy of 0.2 C, i.e., a 0.2 percent increase or decrease in the instrument’s value is negligible. But if the reading is more than 0.2º C, it is considered a high-value error.

Accuracy as a percentage of the true value

The accuracy of the instruments is calculated by identifying and summing the total measurement error every 0.2 percent from the true value. The accuracy determines how much the measured values differ from their true values within a specific range.

Precision Measurement

It can be thought of as a measurement’s relative error. For example, imagine measuring five different objects four times and getting 7.2 kg on each measurement. While your measurements are very precise, they may not be very accurate since it is likely that the object being measured could be heavier than 7.2 kg or lighter than 7.2 kg. To obtain accurate measurements, you must control your measurement process as tightly as possible while still maintaining precision; where you will repeatedly obtain the same values when making multiple measurements. In this specification, the precision of the system is classified into two types:

• Repeatability
• Reproducibility

Repeatability

Repeatability is a measure of the ability to repeat measurements. It describes the amount of variation that occurs in a measurement system as the conditions, such as position and orientation of the measurement device, change.

Reproducibility

Reproducibility refers to the variation that arises using the same measurement process among different instruments and operators over longer periods.

Difference between Accuracy and Precision Measurements

While both accuracy and precision reflect how close a measurement is to an actual value, accuracy reflects how close a measurement is to the accepted or the known value. Any result that needs to be reliable should be accurate and precise. While accuracy measures how close a measurement is to the true or accepted value, precision reflects its reproducibility, even if it is far from accepted.

Precision refers to consistency, or how reproducible measurement is, even if it does not correspond to the actual value. For example, a level filled with water can be used to measure the height of small objects, such as pencils. Precision is important because it reflects whether this method gives reproducible results even if the measured objects are of different heights. Accuracy refers to correctness and represents measurement accuracy corresponding to the known or accepted value.

Accuracy is the difference between a measured value and a common reference value. The smaller the difference between the two values, the more “accurate” the measurement. Precision is the ability to be reproducible, consistent, and systematic in making measurements – in other words, the ability to produce the same results using the same method.

Conclusion

The point is used to denote a single item, in other words, to indicate one thing, not any other thing. Precision is the quality of being exact; exactness and accuracy are two words that usually mean the same. Accuracy can be poor, but precision cannot. There is a difference between the two, though not dramatic. It depends on how you look at it, just like a big difference between two things with small differences. To understand the difference between them and why it is important to know which you’re dealing with, you need to understand each of them.