Accuracy & Precision

In investigations, "ACCURACY" refers to how close a measurement is to the true value and "PRECISION" indicates how consistent repeated measurements are with each other. We need to make sure that when we carry out our investigations, the measurements we take are as accurate and precise as possible. 

• Accurate measurements means that the measurements taken are very close to the "real" value.

• Precise measurements means that the measurements taken are close together. 

Accurate results allow for accurate conclusions to be made.

Errors in the way we measure directly impact both the accuracy and precision of a result. The more errors present in a measurement, the less accurate and precise the data will be. 

The Two Types of Errors

There are two types of error that can occur when you are making measurements. Being aware of these errors is the first step to eliminating them and ensuring the validity of your results.

• Systematic errors occur when taking measurements. If the measuring instrument is not calibrated correctly, or if you make a particular mistake every time you take the measurement, then the measurements will be consistently incorrect throughout the experiment.

• Random error is always present in a measurement. Random errors are due to unpredictable fluctuations in the equipment or inconsistencies in the interpretation of the readings. It is much easier to notice random errors because two readings for the same measurement will appear as widely different numbers in the data.

Systematic errors are not easy to spot because they do not appear as a single difference in the data set. These errors can be avoided by making sure that the measurements are accurate and by repeating the experiment to show any errors.

Choose the most accurate measuring equipment

Measuring equipment with more decimal places or divisions in the scale generally allow you to take more accurate readings.

• For example if you wanted to measure the mass of a tomato you could use a kitchen scale that gave the mass as 106 g, but when you used a science scale, that could give a reading to 2 decimal places, you got 105.55 g. The science scale is more accurate. 

• Another example is using a beaker to measure 233 mL of water compared to using a measuring cylinder. The measuring cylinder is more accurate because it has smaller graduations (lines for each measurement) than the beaker.

Avoid Zero Error

To make accurate measurements you need to use the equipment correctly. This includes starting measurements from zero to avoid ZERO ERROR. 

• For example you want to measure out 10 grams of salt. You place a beaker on the scale and press tare. Pressing the "Tare" button zeroes the scale so the scale is not measuring the mass of the beaker anymore. Then you add the salt until the display reads 10 grams. 

• Another example is making sure, when using a ruler, that you start measuring at zero, not the start of the ruler (there is often a gap between the start of the ruler and zero). 

Avoid Parallax Error

To make accurate measurements you need to use the equipment correctly. This includes lining up your line of sight so that it is perpendicular to the scale to avoid PARALLAX ERROR

For example, look straight ahead at the measuring cylinder or ruler to make sure your eye is level with the scale. Do not tilt your head or body. 

Sometimes you may want to improve your accuracy by repeating measurements. For example, you measure the temperature of tap water three times and obtain the following results: 18.9°C, 18.6°C and 18.7°C. You only want one measurement, so you calculate the average. To do this you: