Year	Number
1997	23.30221
1998	23.40996
1999	24.04234
2000	24.23563
2001	24.62907
2002	24.89608
2003	24.42921
2004	25.00517
2005	24.30839
2006	24.15715
2007	23.99916
2008	23.95168
2009	23.01603
2010	24.34106
2011	23.49184
2012	24.11542
2013	22.74308
2014	22.42943
2015	22.25193
2016	22.33155
2017	21.52737
2018	22.18366