# Accuracy vs. Precision: Key Concepts in Measurement

#### ByChemistry Fundamentals

Sep 25, 2023

Accuracy and precision are two important concepts in measurement and data analysis, often used in various fields such as science, engineering, and statistics. They describe how close measurements or data points are to a true value or to each other. Here’s how accuracy and precision are defined, along with suitable examples:

1. Accuracy:

Accuracy refers to the degree of closeness between a measured or observed value and the true or accepted value. In other words, it measures how well a measurement represents the actual or expected value. Accuracy is often expressed as a percentage or a fraction, where a higher value indicates a more accurate measurement.

Example:

Imagine you have a target in a shooting range, and you are trying to hit the bullseye. If your shots consistently land near the center of the target, your shooting is accurate. If the shots cluster around the bullseye, even if they are not all in the exact center, you are still accurate because you are close to the true target.

2. Precision:

Precision, on the other hand, refers to the degree of consistency or repeatability in a series of measurements. It measures how closely individual measurements or data points agree with each other. A highly precise measurement has very little variation between individual measurements, even if they are far from the true value.

Example:

Let’s say you have a set of scales, and you are weighing a bag of sugar multiple times. If each time you get a weight close to the actual weight (e.g., 1 kg), you have accuracy. However, if the weights you get are consistently around the same value (e.g., 0.95 kg, 0.96 kg, 0.94 kg), even though they are not exactly 1 kg, you have precision because your measurements are close to each other.

To summarize, accuracy is about how close measurements are to the true value, while precision is about how close multiple measurements are to each other. It’s possible to have measurements that are accurate but not precise (close to the true value but with a lot of variation), precise but not accurate (consistent but consistently off-target), or both accurate and precise (close to the true value and tightly clustered together).

Accuracy and precision are two distinct concepts used to evaluate the quality of measurements or data. They describe different aspects of how close measurements are to a true or expected value and how close multiple measurements are to each other. Here’s a summary of the key differences between accuracy and precision:

1. Definition:
• Accuracy refers to the degree of closeness between a measured or observed value and the true or accepted value. It evaluates how well a measurement represents the actual value.
• Precision refers to the degree of consistency or repeatability in a series of measurements. It assesses how closely individual measurements or data points agree with each other.
2. Focus:
• Accuracy focuses on the relationship between the measured value and the true value. It answers the question, “How close is the measurement to the actual or expected value?”
• Precision focuses on the consistency of measurements among themselves. It answers the question, “How closely do repeated measurements agree with each other?”
3. Measurement Error:
• Accuracy is related to systematic errors, which are consistent and tend to shift measurements away from the true value. Inaccurate measurements result from bias or calibration issues.
• Precision is related to random errors, which introduce variability or scatter in measurements. Precise measurements have low random error because they cluster closely together.
4. Representation:
• Accuracy is often expressed as a measure of the difference between the measured value and the true value, typically in the form of a percentage or a fraction.
• Precision is typically quantified by the spread or standard deviation of a set of measurements. It is not directly expressed as a percentage.
5. Example:
• For example, if you are using a ruler to measure the length of a pencil and your ruler consistently reads 0.5 cm longer than the true length of the pencil, your measurements are accurate but not precise because they are consistently off-target.
• If you take multiple measurements of the same object with a ruler, and the measurements vary slightly but cluster closely together around the same value, your measurements are precise but not necessarily accurate if they are consistently offset from the true value.

In summary, accuracy is about how close measurements are to the true value, while precision is about how close measurements are to each other. Ideally, good measurements should be both accurate (close to the true value) and precise (consistent and closely clustered together). However, it’s possible to have measurements that are accurate but not precise or precise but not accurate, depending on the sources of error and variability in the measurement process.