# Level 3

## 91583: Conduct an experiment to investigate a situation using experimental design principles

Updated May 2019. The sections dealing with ‘Posing an investigative question about a given experimental situation’, ‘Making an appropriate formal statistical inference’ and ‘Required quality of student response’ have been updated.

Students need to provide evidence of each component of the investigation process detailed in Explanatory Note 3 of the standard.

### Posing an investigative question about a given experimental situation

Sufficient time needs to be allocated for students to research the context and acquire appropriate and relevant contextual knowledge. For all grades, students need to identify a purpose and develop an investigative question which is informed by this contextual knowledge. The investigative question needs to be about the effect of the treatment that is used in the experiment.

An appropriate investigative question would be: ‘Does being blindfolded have an effect on how long you can balance on one leg? I am going to investigate this for my year 13 Statistics class at XXX College’.

### Planning the experiment using experimental design principles

The plan needs to identify the experimental units, the treatment and response variables, and how the experimental units are randomly allocated to the treatment and control groups. It is not appropriate for any experimental unit to be a member of both the treatment and control groups.

### Making an appropriate formal statistical inference

The formal statistical inference needs to be a causal inference based on the strength of the evidence from the re-randomisation of the experimental data.

An appropriate causal inference would be:

‘As the tail proportion is less than 10% this tells me that the observed difference in the median length of time the students in my year 13 Statistics class at XXX College balance on one leg with and without a blindfold is unlikely when chance is acting alone.

Because the students were randomly assigned to the groups we may claim that wearing a blindfold was effective in lowering the balancing time for these students. The students were not randomly selected so we need to consider carefully as to which wider groups the conclusions may apply’.

### Required quality of student response

For Merit, students need to justify all findings with reference to evidence from the experiment and link the purpose and findings to their research.

For Excellence, students need to integrate the statistical and contextual knowledge gained from their research throughout the response. This may include reflecting on the process, discussing how possible sources of variation are dealt with during the design phase, or considering other variables.