Year	Number
1970	11.82741
1971	12.46361
1972	12.41378
1973	13.93838
1974	16.23969
1975	16.86851
1976	16.49536
1977	17.33145
1979	15.84985
1980	19.36982
1981	17.29043
1982	17.117
1983	17.34521
1984	16.86941
1985	17.32376
1986	17.92618
1987	17.99547
1988	17.71439
1989	17.37097
1990	16.22225
1991	16.61269
1992	16.34042
1993	14.35511
1994	13.85273
1995	13.64093
1996	14.28642
1997	14.47057
2002	14.80908
2017	19.25267