Precision and Accuracy

Precision and Accuracy:

The agreement of the measured value with the true value of the measured quantity is called accuracy. If the measurement of a dimensions of a part approximates very closely to the true value of that dimension, it is said to be accurate. Thus the term accuracy denotes the closeness of the measured value with the true value. The difference between the measured value and the true value is the error of measurement. The lesser the error, more is the accuracy.

Precision, The terms precision and accuracy are used in connection with the performance of the instrument. Precision is the repeatability of the measuring process. It refers to the group of measurements for the same characteristics taken under identical conditions. It indicates to what extent the identically performed measurements agree with each other. If the instrument is not precise it will give different (widely varying) results for the same dimension when measured again and again. The set of observations will scatter about the mean. The scatter of these measurements is designated as , the standard deviation. It is used as an index of precision. The less the scattering more precise is the instrument. Thus, lower, the value of , the more precise is the instrument.

Accuracy: Accuracy is the degree to which the measured value of the quality characteristic agrees with the true value. The difference between the true value and the measured value is known as error of measurement.

Distinction between Precision and Accuracy

Accuracy is very often confused with precision though much different. The distinction between the precision and accuracy will become clear by the following example. Several measurements are made on a component by different types of instruments (A, B and C respectively) and the results are plotted. In any set of measurements, the individual measurements are scattered about the mean, and the precision signifies how well the various measurements performed by same instrument on the same quality characteristics agree with each other.

​The difference between the mean of set of readings of the same quality characteristic and the true value is called as error. Less the error more accurate is the instrument.

Figure shows that the instrument A is precise since the results of number of measurements are close to the average value. However, there is a large difference (error) between the true value and the average value hence it is not accurate.

The readings taken by the instruments are scattered much from the average value and hence it is not precise but accurate as there is a small difference between the average value and true value.

Figure shows that the instrument is accurate as well as precise.

Factors affecting the accuracy of the measuring system:

The basic components of an accuracy evaluation are the five elements of a measuring system such as:

1. Factors affecting the calibration standards

2. Factors affecting the workpiece

3. Factors affecting the inherent characteristics of the instrument

4. Factors affecting the person, who carries out the measurements, and 5. Factors affecting the environment.

1. Factors affecting the standard. It may be affected by: a. Coefficient of thermal expansion,

​b. Calibration interval,

c. Stability with time,

d. Elastic properties,

e. Geometric compatibility

2. Factors affecting the Workpiece, these are:

a. Cleanliness, surface finish, waviness, scratch, surface defects etc., b. Hidden geometry,

c. Elastic properties,

d. Adequate datum on the workpiece

e. Arrangement of supporting workpiece

f. Thermal equalization etc.

3. Factors affecting the inherent characteristics of Instrument a. Adequate amplification for accuracy objective, b. Scale error,

c. Effect of friction, backlash, hysteresis, zero drift error,

d. Deformation in handling or use, when heavy workpieces are measured e. Calibration errors,

f. Mechanical parts (slides, guide ways or moving elements) g. Repeatability and readability

h. Contact geometry for both workpiece and standard 4. Factors affecting person:

a. Training, skill

b. Sense of precision appreciation,

c. Ability to select measuring instruments and standards d. Sensible appreciation of measuring cost,

e. Attitude towards personal accuracy achievements

f. Planning measurement techniques for minimum cost, consistent with precision requirements etc

5. Factors affecting Environment:

a. Temperature, humidity etc.,

b. Clean surrounding and minimum vibration enhance precision, c. Adequate illumination

d. Temperature equalization between standard, workpiece, and

instrument, e. Thermal expansion effects due to heat radiation from lights, heating

elements, sunlight and people, f. Manual handling may also introduce thermal expansion.

Higher accuracy can be achieved only if, all the sources of error due to the above five elements in the measuring system are analysed and steps taken to eliminate them.

The above analysis of five basic metrology elements can be composed into the acronym.

SWIPE, for convenient reference Where, S – STANDARD W WORKPIECE I INSTRUMENT P PERSON E ENVIRONMENT