| No | Pattern | Freq | Relations |
|---|---|---|---|
| 1 | [verb] [prep] a long period of | 31 |
|
| 2 | [vb pp] [prep] a long period of | 27 |
|
| 3 | [verb] over a long period of | 15 |
|
| 4 | [verb] for a long period of | 8 |
|
