DIFFERENCE BETWEEN ACCURACY AND PRECISION AND ITS IMPACT ON ENGINEERING DECISIONS

There is a difference between the two terms “Precision” and “Accuracy”, although sometimes these are used interchangeably; however for engineers, it is necessary to understand the difference between the two as it can have significant impact on their decisions.

Definitions:

Precision is the quality of being exact, or degree of exactness, while,

Accuracy is the quality of being correct, or measure of correctness.

Now let us discuss these terms in some detail.

Precision:

The term “Precision” is used in two contexts. When we say an instrument is precise to a certain degree, it means that it can report results precisely, to that degree. For example, a foot ruler is precise to 1 mm, as it can report measurements precisely to a millimeter. A vernier caliper is precise to 0.1 mm, while a micrometer is precise to 0.01 mm. Note that regarding ruler the degree of precision is 1 mm not 1.0 mm. Although they are both the same but 1.0 mm implies that the ruler is precise to first decimal of mm, whereas this is not the case, a foot ruler cannot report results to the first decimal of mm. Therefore, when we talk about instruments and devises, the term “Precise” implies their degree of exactness in reporting "Individual Results".

The second context in which the term “Precision” is used is for the instrument to have the ability to report same results consistently. We can assess the precision of an instrument by repeatedly measuring the same quantity several times and comparing the results. If we take the example of an EDM device, we know that it is precise to 1mm, but what is its precision regarding ability to reproduce same result? To find that out we can setup the EDM devise and prism at two established points and measure the distance. Suppose the distance reported is 324.556 m. Now if we take several more readings for the same two points and get the following results: 324.555, 324.557, 324.556, 324.555, 324.554, 324.558 we will know that the instrument has high precision (results are very near to each other). However, for another EDM devise, if the results are like this: 324.587, 324.543, 324.524, 324.594, 324.561, 324.551, it is clear that this instrument does not have high precision (results are far apart) as compared to the previous one. We can apply statistical method (Standard Deviation etc.) to get a quantitative comparison of precision of various instruments. Here, precision refers to ability of the instrument to produce consistent results "over a number of measurements" as opposed to Individual results in the previous case.

Precision of a process or a procedure can also be defined in this way based on the fact whether it produces consistent results over the number of times that it is applied on, or not.

Summarizing the above discussion, we should know that an instrument can be precise (to a high degree) but still may not have precision (of consistency). Hence, whenever a decision is required regarding purchasing an instrument, may it be Total Station, Laboratory Equipment or Concrete or Asphalt Batching Plants etc., one should study both of these aspects before making a final decision.

One important point which needs to be kept in mind regarding precision is that the instrument is only as precise as the degree of precision of all its individual components. Hence, for a concrete batching plant all the load cells for coarse and fine aggregates and cement should be of same precision otherwise overall precision of the plant cannot be guaranteed. If one of the load cells is precise to 10kg while all others are precise to 1 kg, then batching plant cannot claim an overall precision of less than 10kg. (The issue of how to calculate system precision from component precision will be dealt sometime later, in sha Allah)

Another point to keep in mind is that the display (to a number of decimal places) of an instrument does not necessarily mean that it is precise to that degree. So a Total Station company may claim a 1” precision for their instrument, and when you check the display on the total station it may show angle readings to a precision of 1”, but in reality this might not be the case. The internal mechanism of the Total Station might not be able to give such a precision. In reality its precision might be 5” due to low standard of internal mechanism. In this case proper calibration of the instruments is necessary to reveal its true precision. These points should be kept in mind while selecting instruments for your projects.

Accuracy:

As far as accuracy is concerned, it is related more to the environment in which the instrument is being used (user and other environmental factors) rather than the instrument itself. Accuracy can be achieved by adopting correct procedure and applying necessary corrections.

Considering the example of a ruler, we know that it gives a precision of 1 mm but if the user is inexperienced and does not remove the parallax error or check zero correction (matching the zero of ruler with the start of the line); his measurement can be off by several millimeters. In this case although the user is reporting the measurement to nearest millimeter but his measurement is not accurate because it is not correct, due to the above mentioned errors.

Consider another example, if we want to measure dimensions of a building and the instrument at our disposal is the foot ruler, we know that our measurement will not be very accurate. This is due to the fact that we have to move the ruler many times and this will induce errors in the total measurement, thereby producing inaccurate result. To overcome this type of error, we can make a technological leap and use a steel tape of sufficient length to measure the dimension of building in one stretch. This will remove the error of moving the measuring device. However, steel tape has its own set of environmental factors which can affect accuracy. So, we have to be careful to apply just the right amount of tension (pull) to the tape, and apply the temperature and sag correction to get accurate results. If we want to get more accurate result, we can use an EDM device, but here too we have to consider the environmental factors which affect the results of EDM device, and apply necessary corrections and adopt correct procedures to get accurate results. Hence, as far as applying necessary corrections are concerned, applying temperature, pressure, spherical and prism corrections etc. are necessary to get accurate results from an EDM device. While, as far as adopting correct procedures is concerned, then for example, the user should know that EDM device gets affected by high tension lines and vibration of passing heavy traffic, so to get accurate results these points should be addressed.

It should be noted that no matter how highly precise an instrument we select, it will always be affected by the environmental factors and an engineer should know how to deal with them if accurate results are required.

From the above discussion it should be clear that accuracy depends largely on the user (and other environmental factors) rather than the instrument. So even a highly precise instrument can give completely inaccurate results because of the way it is used (or misused). Therefore, keep in mind the fact that just because you have purchased a very expensive and precise instrument does not necessarily mean you will get accurate results. It is imperative for the user of the instrument to be well trained, and for him to adopt correct procedure and apply necessary corrections to get accurate results.

The purpose of the article was to clarify the difference between Precision and Accuracy, and its impact on engineering decisions.

Summary of the above discussion is shown in the Table below. Any feedback on this is most welcome.