A researcher wanted to investigate whether pre-sleep screen use impacts reported sleep quality. To do this, she conducted an independent measures experiment. Group 1 read a physical book for 30 minutes before sleeping, while Group 2 used a smartphone for 30 minutes before sleeping. On waking, participants rated their sleep quality on a scale of 1 ('very restless') to 15 ('perfectly refreshed').
| Ratings of sleep quality (1–15) | |
|---|---|
| Physical book group | Smartphone group |
| 12 | 6 |
| 14 | 4 |
| 15 | 9 |
| 8 | 5 |
| 13 | 12 |
| 10 | 11 |
| 9 | 5 |
| 14 | 3 |
| 11 | 6 |
| 12 | 7 |
Explain how you would find the critical value to compare the calculated value to after conducting this test.
