VCE Physics requires that students can distinguish between and apply the terms ‘accuracy’, ‘precision’, ‘repeatability’, ‘reproducibility’, and ‘validity’ when analysing their own and others’ investigation findings. An understanding of the terms ‘accuracy’ and ‘precision’ is also important in the analysis and discussion of investigations of a quantitative nature.
A measurement result is considered to be accurate if it is judged to be close to the ‘true’ value of the quantity being measured. The true value is the value (or range of values) that would be found if the quantity could be measured perfectly. For example, if an experiment is performed and it is determined that a given substance had a mass of 2.70 g, but the true value of mass is 3.20 g, then the measurement is not accurate since it is not close to the true value. The difference between a measured value and the true value is known as the ‘measurement error’.
‘Accuracy’ is not a quantity and therefore cannot be given a numerical value. It is allowable for a measurement to be described as being ‘more accurate’ when its method and/or instruments clearly reduce measurement error, such as using a triggered electronic timer system compared to a hand-operated stopwatch. Accuracy may not be quantified: ‘measurement error’ is the quantity used to evaluate how close a measured value is to the true value.
Experimental precision refers to how closely two or more measurement values agree with each other. A set of precise measurements will have very little spread about their mean value. For example, if a given substance was weighed five times, and a mass of 2.70 g was obtained each time, then the experimental data are precise. Precision gives no indication of how close the results are to the true value and is therefore a separate consideration to accuracy, so that if the true mass in the above example was 3.20 g then these data are precise but inaccurate.
Quantitatively, a measure of precision would be a measure of spread of measured values.
A measured mass of 2.7 g ± 0.1 g is less precise than 2.702 g ± 0.001 g.
Replication of procedures: repeatability and reproducibility
Experimental data and results must be more than one-off findings and should be repeatable and reproducible to draw reasonable conclusions. Repeatability refers to the closeness of agreement between independent results obtained with the same method on identical test material, under the same conditions (same operator, same apparatus and/or same laboratory). Reproducibility refers to the closeness of agreement between independent results obtained with the same method on identical test material but under different conditions (different operators, different apparatus and/or different laboratories). The purposes of reproducing experiments include checking of claimed precision and uncovering of any systematic errors that may affect accuracy from one or other experiments/groups. Experiments that use subjective human judgment/s or that involve small sample sizes or insufficient trials may also yield results that may not be repeatable and/or reproducible.
A measurement is ‘valid’ if it measures what it claims to be measuring. Both experimental design and the implementation should be considered when evaluating validity. Both experimental design and the implementation should be considered when evaluating validity. Data are said to be valid if the measurements that have been made are affected by a single independent variable only. They are not valid if the investigation is flawed and control variables have been allowed to change or there is observer bias.