The Sleep and Memory Internal Assessment
A student designs a school-based investigation for the SACE Stage 2 Psychology internal assessment. The hypothesis is that students who report sleeping at least 8 hours the night before a test will score higher on a 20-item recall quiz than students who report sleeping fewer than 6 hours. The student surveys one Year 12 class of 22 students and finds 9 in the 8-hour group and 13 in the 6-hour group. The quiz is administered at 9:00 am on a Monday. Three students in the short-sleep group report having consumed energy drinks that morning. The student wants to conclude that sleep duration causes better recall and writes that the results are generalisable to all SACE students.
1. Identify the independent and dependent variables and state the operational definition of each. Explain one strength and one limitation of these operational definitions.
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Independent variable: reported sleep duration the night before the test, operationalised as self-reported hours of sleep grouped into two categories: at least 8 hours versus fewer than 6 hours. Dependent variable: recall performance, operationalised as the score out of 20 on a 20-item recall quiz administered at 9:00 am on a Monday.
Strength: the dependent variable is operationalised numerically as a quiz score out of 20, which allows objective comparison between groups and permits calculation of a mean and standard deviation. This makes the data quantitative and reduces ambiguity about what "better recall" means.
Limitation: sleep duration is operationalised through self-report rather than objective measurement such as actigraphy. Students may misjudge or misreport their sleep, and the categorical split (8+ versus under 6) discards the 6-to-8-hour range, so participants in that middle band are excluded and the measure may not accurately reflect actual sleep. This weakens the validity of the independent variable.
2. The student claims the results show causation and generalise to all SACE students. Evaluate both claims using the details of the design.
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Causation claim: this is not supported. The design is a quasi-experiment using pre-existing groups, not random allocation, so the groups may differ on confounding variables. The three students in the short-sleep group who consumed energy drinks introduce a plausible alternative explanation for any difference in recall, since caffeine can affect attention and arousal. Other confounds include motivation, prior knowledge of the material, and test anxiety. Without controlling these, a correlation between sleep group and quiz score cannot establish that sleep duration caused the difference.
Generalisability claim: this is also weak. The sample is one Year 12 class of 22 students, a small and non-random convenience sample from a single school. The results cannot be assumed to represent all SACE students, who vary in year level, subject load, and school context. Additionally, the exclusion of the 6-to-8-hour group means the sample does not represent typical sleep patterns. The appropriate conclusion is that the study suggests an association within this class that warrants further investigation with random allocation and objective sleep measurement.