Level 2


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91168: Carry out a practical physics investigation that leads to a non-linear mathematical relationship

Updated December 2017. This document has been updated in its entirety to address new issues that have arisen from moderation.

The purpose of the investigation is to identify a non-linear mathematical relationship between two variables. Data collected should be sufficient to allow confident identification of the non-linear relationship. The number of values of the independent variable will be at least five.

Assessment may involve a single event or a portfolio of experiments, each of which is a complete investigation. Grades for each investigation will be decided using professional judgement based on holistic examination of the evidence using criteria from the Achievement Standard.

Practical Investigation

Learners may be provided with an aim and an outline of a method but not a template. Learners can gather data in pairs, but each learner should be involved in making measurements. Once data has been gathered, group discussion or teacher input is not appropriate. Assessors should be able to verify the evidence submitted for assessment is authentic and has been produced by the learner.

Evidence for units for gathered data and processed data can come from a results table or labelled axes of a graph. There is no requirement to define which variable is the independent and which the dependent.


Learners may use graph paper and/or digital technology to analyse data. An appropriate graph is correctly labelled, has correctly plotted points and a best fit straight line that is a reasonable fit to the plotted points. Evidence for the conclusion includes a proportionality in terms of the variables (not y and x) or a straight-line graph, that shows the transformed variable on the axis label.

Interpretation of evidence

For Merit, a control variable has a significant effect on the results. Describing control variables that would not affect the results does not contribute evidence. Evidence of techniques such as repeating and averaging might be in the results table. Data points can only be used to calculate the gradient if precisely on the line. The relationship equation should be stated in terms of the variables being investigated.

For Excellence, when describing accuracy improving techniques, how these made measurements more reliable should be explained. Any reasons given, for the limits of the value of the independent variable, or descriptions of difficulties, should be related to the reliability of measurements rather than limitations of the supplied equipment. Overall, learners should have the equivalent of two good discussion points.

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