| Time base (years) | Time periods | Mann Whitney U | Median frequency |
| 10 | 1780-1909 | 21.5 sig 2% | 9 |
|
| 1910-2019 |
| 3.0 |
| 30 | 1780-1899 | 0.5 sig > 0.2% | 29.5 |
|
| 1900-2019 |
| 15.5 |
| 10 | 1780-1959 | 17.5 sig 2% | 9.0 |
|
| 1960-2019 |
| 3.5 |
| 30 | 1780-1959 | 0.0 sig > 0.2% | 28 |
|
| 1960-2019 |
| 13 |