Accuracy and precision facts for kids

Accuracy and precision are two important ideas in science and everyday life. They help us understand how good our measurements are.

• Accuracy means how close your measurement is to the real or true value. Imagine you are throwing darts at a target. If your darts land very close to the center, you are accurate.
• Precision means how close your measurements are to each other. If you throw darts and they all land in a tight group, even if it's not the center, you are precise.

Sometimes, people use these words to mean the same thing, but in science, they have different and very specific meanings.

What do Accuracy and Precision Mean?

Accuracy is how close your measurements are to the true value. Precision is how close your repeated measurements are to each other.

In science and engineering, accuracy tells us how close a measurement is to the actual, correct value. Precision tells us how close repeated measurements are to each other, even if they are not close to the true value.

Think of it like this:

• If you weigh a bag of sugar that is supposed to be 1 kilogram, and your scale says 1.001 kg, that's very accurate.
• If you weigh the same bag five times, and your scale shows 1.001 kg, 1.002 kg, 1.001 kg, 1.000 kg, and 1.002 kg, your scale is also very precise because the readings are all very close together.

A measurement system can be:

• Accurate but not precise: Your measurements are close to the true value, but they are spread out.
• Precise but not accurate: Your measurements are very close to each other, but they are all far from the true value. This can happen if there's a problem with your measuring tool.
• Neither accurate nor precise: Your measurements are all over the place and far from the true value.
• Both accurate and precise: This is the best outcome! Your measurements are close to the true value and also close to each other.

For example, if you are using a ruler that is slightly too short, all your measurements might be precise (close to each other) but not accurate (they will all be a bit off from the true length). This is called a systematic error.

Measuring Accuracy and Precision

In science, we often want to know how accurate and precise our tools and methods are. We do this by measuring things that have a known, exact value. These are called standards.

For example, if you want to check if a thermometer is accurate, you might use it to measure the boiling point of water, which is known to be 100 degrees Celsius at sea level. If your thermometer reads 98 degrees every time, it's precise but not accurate. You would need to calibrate it.

When you repeat measurements and then average them, the average measurement tends to be more accurate. This is why scientists often take many readings and then calculate the average.

Significant Figures

One way to show how precise a measurement is, especially when you don't state the error directly, is by using significant figures. These are the digits in a number that carry meaning and show how accurate the measurement is.

For example:

• If you write 843.6 meters, it means you measured it very carefully, possibly to the nearest tenth of a meter.
• If you write 843 meters, it means you measured it to the nearest meter.

If you write 8,000 meters, it's not clear how precise the measurement is. To avoid confusion, scientists often use scientific notation, like 8.0 × 10³ meters. This shows that the measurement is precise to the nearest hundred meters.

Types of Precision

Precision can be broken down into two parts:

• Repeatability: This is how close measurements are when the same person uses the same tool in the same way over a short period.
• Reproducibility: This is how close measurements are when different people use different tools, or the same tool at different times, to measure the same thing.

ISO Definition (ISO 5725)

According to ISO 5725-1, accuracy includes both trueness (how close to the true value) and precision (how close measurements are to each other).

The ISO (a group that sets worldwide standards) has a specific way of defining accuracy and precision.

According to ISO 5725-1, the general term "accuracy" describes how close a measurement is to the true value. When we talk about a group of measurements, accuracy includes both random errors (which affect precision) and systematic errors (which affect how close the average is to the true value).

ISO also uses the term trueness to describe how close the average of a set of measurements is to the actual true value. So, for ISO, accuracy is a combination of trueness and precision.

In Information Systems

In areas like databases and web search engines, accuracy is used differently. Here, it often means how well a system correctly finds or doesn't find what you're looking for.

For example, if you search for "dogs" and the search engine shows you 100 results, and 90 of them are actually about dogs, then the system has a high level of accuracy for that search.

Other terms like precision and recall are also used here:

• Precision (in this context) is the fraction of the results that are actually relevant to your search.
• Recall is the fraction of all the relevant items that the system actually found.

These ideas help us understand how good a search engine or information system is at giving us the right information.

See also

In Spanish: Precisión y exactitud para niños

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