Year	Number
1971	39.52345
1973	38.53942
1974	36.29005
1975	36.93395
1976	37.38012
1977	35.50408
1978	35.58112
1979	34.38803
1980	33.92576
1981	33.22538
1982	32.51091
1983	31.80293
1984	30.74945
1985	29.54059
1986	28.48781
1987	27.66025
1988	26.68086
1989	26.28504
1990	25.7385
1991	24.8264
1992	23.35497
1993	20.87476
1994	19.8356
1995	19.83518
1996	19.26957
1998	17.92081
2013	13.91752
2014	14.16012
2015	14.0399
2016	13.59595
2017	13.44283