An environmental scientist wants to investigate if there is a correlation between the concentration of dissolved oxygen in a river and the distance from a local wastewater treatment facility.
The scientist randomly selects 18 testing sites along the river. At each site, they measure the distance from the facility (in km) and the dissolved oxygen concentration (in mg/L).
The scientist proposed a directional (one-tailed) hypothesis, predicting that the dissolved oxygen concentration would be positively correlated with the distance from the facility.
The scientist chose to use the 5% level of significance and calculated a Spearman's rho correlation coefficient of 0.438.
Table 1 Critical values of Spearman's rho (rsr_srs)
| Level of significance for a one-tailed test | 0.05 | 0.025 |
|---|---|---|
| Level of significance for a two-tailed test | 0.10 | 0.05 |
| N = 17 | 0.414 | 0.485 |
| N = 18 | 0.401 | 0.472 |
| N = 19 | 0.391 | 0.460 |
| N = 20 | 0.380 | 0.447 |
| N = 21 | 0.370 | 0.435 |
Note: The calculated value of rho must be greater than or equal to the critical value to be significant.
Identify the appropriate critical value from Table 1. Explain your choice.
121 exam-style questions on AQA A Level Psychology Inferential testing (A-level only). Each one has a worked solution and a mark scheme showing where the marks go.