Precision and Accuracy in Science

When we undertake practical work in science, being able to evaluate the experimental method and the quality of the collected data is a crucial skill. In any science subject, the concepts of precision and accuracy are very important and knowledge of them is required at both GCSE and A Level.

In this article, I’ll break down everything you need to know about accuracy and precision and share my top tips for answering practical exam questions

Understanding precision and accuracy in science: what are they, and why do we use them?

In science, both accuracy and precision are crucial for obtaining reliable data and drawing valid conclusions.

Accuracy refers to how close a measured value is to the true or accepted value of a quantity. 

A high level of accuracy ensures the data is

• Valid - this means that the data is trustworthy and can be used to make predictions or draw conclusions

• Low in systematic errors - this means there are little to no flaws in the experimental method or apparatus used

Precision, on the other hand, refers to how close the measured values are to each other when repeated measurements are made.

A high level of precision ensures the data is

• Reliable - this means that the measurements could be reproduced consistently

• Low in random errors - this means there is little to no spread of results about mean values

It is possible for results to be precise but not accurate, and it is also possible for results to be accurate without being precise. This can be visualised using darts on a dartboard. Think of the central bullseye as the true value. The closer the darts land to the bullseye, the greater the accuracy. 

The darts are…

• Accurate and precise if they land close to the bullseye and close together

• Accurate but not precise if they are all spaced equally around the bullseye

• Precise but not accurate if they all land close together, but far from the bulls-eye

• Neither accurate nor precise if they are neither close to the bullseye, nor close to each other

Reducing random and systematic errors

Measurements of quantities are made with the aim of finding the true value of that quantity. In reality, it is impossible to obtain the true value of any quantity as there will always be a degree of uncertainty. The uncertainty is an estimate of the difference between a measurement reading and the true value. As a science student, you must be aware that every measurement has an inherent uncertainty. 

The two types of measurement errors that lead to uncertainty are random errors and systematic errors.

Random errors are when you get a large variation in readings about a mean value when you take a measurement repeatedly. These affect the precision of the measurements. To reduce random errors, you should repeat the measurements as many times as possible and exclude anomalous results.

Systematic errors are when there is a fault with a measuring device or method. These affect the accuracy of the measurements. To reduce systematic errors, you need to find out what is causing it and correct your measurements accordingly. This is usually easiest to spot once the data has been processed, such as when it is plotted on a graph, or when a final value is calculated. 

One example of a systematic error when investigating osmosis in biology is in the measurement of the length of a potato cylinder with a ruler. If the student measures all the lengths from the edge of the rule instead of the zero mark, then all of the measurements will be incorrect by the same amount.

How are precision and accuracy assessed in exams?

When it comes to GCSE and A Level exams in Physics, Chemistry, and Biology, you will be required to demonstrate your understanding of accuracy and precision through a combination of practical experiments, data analysis, and evaluation of experimental results. 

At GCSE level, you will gain lots of valuable experience carrying out experiments but you will not be directly assessed by your teacher. Whereas, at A Level you will likely have a practical skills assessment element to your qualification, so make sure to check your specification to ensure you are familiar with what it entails. 

All GCSE and A Level science exams will include some questions in written exam papers that require you to apply your knowledge of practical skills, such as 

• identifying sources of error

• calculating uncertainties

• plotting graphs and drawing lines of best fit

• identifying patterns and trends in data

• comparing graphs or sets of data

• discussing the reliability of data

• making suggestions to improve accuracy and precision

Key differences between the sciences at A Level

Practical work across Physics, Chemistry and Biology is designed to be assessed as consistently as possible, however, the nature of each subject means there are some key differences, particularly at A Level.

Physics

In Physics, one of the key techniques students have to master is the implementation of methods to increase the accuracy of measurements. 

This can be achieved in a number of ways, such as 

• Taking measurements of time or length over multiple intervals

• Using fiduciary markers, set squares or plumb lines 

For example, when measuring the fringe spacing of an interference pattern, students would be expected to measure multiple fringe spacings and then divide by the number of fringe spacings. A fringe spacing is a very small measurement and it is often difficult to see the middle of each bright fringe (as each maxima can be broad and have blurred edges), so measuring the largest possible distance reduces the uncertainty in the measurement.

The same technique can be applied to measuring the time period of an oscillation. It is far more accurate to measure the time for 10 oscillations, then divide by 10, rather than just timing 1 oscillation.

Another method to improve the accuracy of an experimental set-up is by using

• Fiducial marker: used to mark an exact point to take measurements from

• Set square: used to ensure apparatus is aligned parallel or perpendicular

• Plumb line: used to ensure apparatus is vertically aligned

Chemistry

In Chemistry, one of the main practicals students carry out is titration. In titrations, carrying out the procedure once is never sufficient. The experiment must usually be done until there are at least two runs that are concordant i.e. within a certain range, usually 0.10 cm³. 

If runs are not concordant, then it is likely there is a systematic error. This could arise for a number of reasons such as

• Adding the wrong volume of solution from the burette 

• Measuring the wrong amount of solution in the pipette

• Misjudging the end point

Any runs that are not concordant are discarded. This is different to biology practicals where anomalous results are always included unless there is a valid reason not to.

Biology

In Biology, the types of data collected and the graphing skills required are slightly different than in Physics and Chemistry. For example, the large amounts of numerical data produced from ecological studies are very different to the drawings produced from microscope slides of live specimens.

In addition to straight line graphs, students must be aware of the different types of graph, such as

• Scatter graphs

• Jagged-line graphs

• Histograms

• Bar charts

In Biology, it is conventional to refer to straight lines on graphs as curves. This is not the case in Physics or Chemistry.

The type of graphical format used depends on the data

• For qualitative and discrete data, bar charts or pie charts are most suitable

• For continuous data, line graphs or scatter graphs are most suitable

Helpful tips and tricks for revising practical skills

When you are revising practical skills, here are some top tips to keep in mind:

1. Understand the difference between accuracy and precision: Make sure you can differentiate between these two concepts and explain how they are relevant to scientific experiments.

2. Practice data analysis questions: Familiarise yourself with calculating uncertainties, identifying sources of error, and evaluating the reliability of data in practical experiments.

3. Pay attention to units and significant figures: Ensure that you are using the correct units and reporting your results with the appropriate number of significant figures to maintain accuracy and precision.

4. Review past exam questions: Look at previous exam papers to get an idea of how accuracy and precision are assessed in GCSE and A Level science exams and practice answering relevant questions.

5. Revisit practical work: Look back at the notes you took when you carried out practical work. Pay close attention to the experimental procedures, analysis and evaluation of results and any conclusions you drew

By mastering the concepts of accuracy and precision and applying them effectively in practical experiments and data analysis, you can improve your performance in GCSE and A Level science exams and develop essential skills for future scientific endeavours.

Summary

The concepts of accuracy and precision are crucial to understand as a science student, both in written exams and practical work.

Make sure to revise them thoroughly and use the tips and tricks I have given you to ensure that you are fully prepared for any practical skills questions you may encounter.

If you are looking for a stress-free way to revise for your A Level Physics exams, check out our Revision Notes and Topic Questions written by subject specialist teachers and examiners. And sign up to Save My Exams today!

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