| No | Pattern | Freq | Relations |
|---|---|---|---|
| 1 | [prep] a period of | 1599 |
|
| 2 | [prep] a [adj -er] period | 146 |
|
| 3 | a [adj -er] period of | 54 |
|
| 4 | [prep] [adj -er] period of | 20 |
|
