Use bias to evaluate a psychology claim

Psychology SL stimulus: In Perth, Year 12 students are involved in a health project comparing a walking routine with weekly anxiety check-ins. The focus is lunchtime walking and anxious feelings before assessments. Evidence says students in the walking group describe calmer afternoons but exam anxiety still varies. The method uses weekly anxiety scales, walk attendance logs, and reflection notes with 80 participants over 2 weeks. The bias concern is that students who already like exercise may be more likely to volunteer. Use the bias and control map before evaluating the claim. Select the psychology concept, content link, context application, and evaluation limit. Then answer this in your own words: how would this route improve the Psychology SL response for this exact context? AI will analyse your reasoning, evidence use, concept-content-context link, method or ethics judgement, and expression.