4 The effect of water salinity on the growth of a type of grass was studied by a biologist. A random sample of 22 seedlings was divided into two groups \(A\) and \(B\), each of size 11 .
Group \(A\) was treated with water of \(0 \%\) salinity and group \(B\) was treated with water of \(0.5 \%\) salinity. After three weeks the height (in cm) of each seedling was measured with the following results, which are ordered for convenience.
| Group \(A\) | 8.6 | 9.4 | 9.7 | 9.8 | 10.1 | 10.5 | 11.0 | 11.2 | 11.8 | 12.7 |
| Group \(B\) | 7.4 | 8.4 | 8.5 | 8.8 | 9.2 | 9.3 | 9.5 | 9.9 | 10.0 | 11.1 |
Jeffery was asked to test whether the two treatments resulted, on average, in a difference in growth. He chose the Wilcoxon rank sum test.
- Justify Jeffery's choice of test.
- Carry out the test at the \(5 \%\) significance level.