SN4 ""

No	Pattern	Freq
1	[noun] over a period [prep] years	30
2	[noun sg] over a period [prep] years	18
3	[noun pl] over a period [prep] years	10
4	[verb] over a period [prep] years	9
5	[vb pp] over a period [prep] years	9
6	[adv] over a period [prep] years	7
7	[adj] over a period [prep] years	6
8	[adj] over a period [prep] years	6
9	over a [adj] period [prep] years	5
10	over a [adj] period [prep] years	5
11	over a period [prep] [adj] years	109
12	over a period [prep] [num] years	97
13	over a period [prep] five years	12
14	over a period [prep] two years	12
15	over a period [prep] five years	12
16	over a period [prep] two years	12
17	over a period [prep] [det] years	12
18	over a period [prep] three years	8
19	over a period [prep] three years	8
20	over a period [prep] several years	7
21	over a period [prep] several years	7
22	over a period [prep] twenty years	6
23	over a period [prep] twenty years	6
24	over a period [prep] ten years	5
25	over a period [prep] ten years	5
26	over a period [prep] years [prep]	8
27	over a period [prep] years [prep]	8