A cognitive psychologist wanted to investigate whether there was a relationship between cortisol levels (a key physiological marker of stress) and working memory capacity.
An advert was placed on a popular online productivity forum, PeakFocus, asking for working adults between 25 and 45 years old to take part in the study. Eight participants were recruited. They provided a saliva sample to measure their cortisol levels (in micrograms per deciliter) and completed a computerized backward digit-span task to test their working memory capacity (with a maximum possible score of 30).
The results of the study are shown in Table 1 below.
| Participant list | Working memory score /30 | Cortisol level (µg/dL) |
|---|---|---|
| 1 | 24 | 0.45 |
| 2 | 15 | 0.22 |
| 3 | 28 | 0.61 |
| 4 | 11 | 0.15 |
| 5 | 19 | 0.38 |
| 6 | 21 | 0.50 |
| 7 | 14 | 0.28 |
| 8 | 26 | 0.58 |
Explain one limitation of the sampling technique used for this investigation.
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