Why do we need measurement? Measurement is the foundation for all experimental science and technology. Science is the study of nature and we need to use what we see in nature in our calculations. Today we know that the acceleration due to gravity is 9.8 m/s^{2} but would we be able to comprehend that if we had no means of measurement.

Due to presence of universal SI units, we can exchange information with anyone since these units are used worldwide. Measurement is essential for us to understand the external world and through the millions of years of life, we all have developed a sense of measurement. You unconsciously know how much time has passed or how much distance you have walked. The problem here is that the result of every measurement by any measuring instrument contains some **uncertainty**. This uncertainty is referred to as **Error**. Here, there are two terms that are often confused with each other but are not the same, **Accuracy and Precision**.

### What is Accuracy and Precision?

The **closeness of a measured value** to the actual value of the object being measured is called as **accuracy** of a substance. For instance, if in lab you obtain a weight measurement of 3.2 kg for a given substance, but the actual or known weight is 10 kg, then your measurement is not accurate.

The **closeness of two or more measurements** to each other is known as the **precision** of a substance. From the above given example we can figure out that, if you weigh a given substance five times, and get 3.2 kg each time, then your measurement is very precise. Precision is independent of accuracy.

Precision is sometimes separated into:

**Repeatability:** The variation arising when the conditions are kept identical and repeated measurements are taken during a short time period.

**Reproducibility:** The variation arising using the same measurement process among different instruments and operators, and over longer time periods.

In other words, accuracy is the degree of closeness between a measurement and the measurement’s true value. Precision is the degree to which repeated measurements under the same conditions are unchanged.

Figure: Difference between accuracy and precision

A good analogy for understanding accuracy and precision is to imagine a football player shooting at the goal. If the player shoots into the goal, he is said to be accurate. A football player who keeps striking the same goal post is precise but not accurate. Therefore a football player can be accurate without being precise if he hits the ball all over the place but still scores. A precise player will hit the ball to the same spot repeatedly, irrespective of whether he scores or not. A precise and accurate football player will not only aim at a single spot but also score the goal.