Dr Aris wants to investigate the impact of a mindfulness meditation app on stress levels. He measures the physiological stress of 20 participants by recording their salivary cortisol levels (measured in micrograms per deciliter, μg/dL\mu\text{g/dL}μg/dL) immediately before a high-pressure presentation, and then again before a similar presentation after they have used the meditation app for two weeks.
Dr Aris wants to see if there is a significant difference in salivary cortisol levels between the two conditions.
Explain which statistical test Dr Aris should use to analyse his data.
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