*Excludes Observations that spanned multiple years
| Pre 1970 | 5 |
|---|---|
| 1970 | 1 |
| 1971 | 3 |
| 1972 | 0 |
| 1973 | 0 |
| 1974 | 1 |
| 1975 | 2 |
| 1976 | 0 |
| 1977 | 5 |
| 1978 | 4 |
| 1979 | 0 |
| 1980 | 1 |
| 1981 | 1 |
| 1982 | 4 |
| 1983 | 1 |
| 1984 | 7 |
| 1985 | 107 |
| 1986 | 89 |
| 1987 | 155 |
| 1988 | 497 |
| 1989 | 31 |
| 1990 | 62 |
| 1991 | 185 |
| 1992 | 142 |
| 1993 | 242 |
| 1994 | 150 |
| 1995 | 331 |
| 1996 | 180 |
| 1997 | 167 |
| 1998 | 10 |
| 1999 | 133 |
| 2000 | 143 |
| 2001 | 74 |
| 2002 | 39 |
| 2003 | 71 |
| 2004 | 80 |
| 2005 | 70 |
| 2006 | 79 |
| 2007 | 92 |
| 2008 | 104 |
| 2009 | 106 |
| 2010 | 76 |
| 2011 | 82 |
| 2012 | 100 |
| 2013 | 102 |
| 2014 | 111 |
| 2015 | 85 |
| 2016 | 97 |
| 2017 | 109 |
| 2018 | 30 |
| 2019 | 28 |
| 2020 | 16 |
| 2021 | 38 |
| 2022 | 13 |
| 2023 | 3 |
| 2024 | 5 |
| 2025 | 0 |
| Total | 4269 |