| Closed Access | Open Access | Restricted | Unknown | Total | |
|---|---|---|---|---|---|
| 2020 | - | 4 | - | 2 | 6 |
| 2019 | 2 | 163 | - | 68 | 233 |
| 2018 | - | 257 | - | 82 | 339 |
| 2017 | 6 | 62 | 2 | 349 | 419 |
| 2016 | 1 | 53 | 2 | 463 | 519 |
| 2015 | 3 | 46 | 2 | 498 | 549 |
| 2014 | 7 | 40 | 2 | 545 | 594 |
| 2013 | 20 | 46 | 4 | 500 | 570 |
| 2012 | 10 | 42 | 1 | 388 | 441 |
| 2011 | 9 | 41 | 3 | 437 | 490 |
| 2010 | 13 | 31 | 1 | 443 | 488 |
| 2009 | 6 | 15 | - | 431 | 452 |
| 2008 | 4 | 22 | - | 466 | 492 |
| 2007 | 2 | 21 | - | 625 | 648 |
| 2006 | - | 4 | - | 510 | 514 |
| 2005 | - | 13 | - | 428 | 441 |
| 2004 | 2 | 6 | - | 460 | 468 |
| 2003 | - | 6 | - | 446 | 452 |
| 2002 | - | 3 | - | 346 | 349 |
| 2001 | - | 5 | - | 382 | 387 |
| 2000 | - | 5 | - | 324 | 329 |
| 1999 | - | 6 | - | 306 | 312 |
| 1998 | - | 5 | - | 266 | 271 |
| 1997 | - | 2 | - | 177 | 179 |
| 1996 | - | 3 | - | 158 | 161 |
| 1995 | - | - | - | 206 | 206 |
| 1994 | - | 1 | - | 202 | 203 |
| 1993 | - | 1 | - | 153 | 154 |
| 1992 | - | - | - | 65 | 65 |
| 1991 | - | - | - | 13 | 13 |
| 1990 | - | - | - | 10 | 10 |
| 1989 | - | - | - | 10 | 10 |
| 1988 | - | - | - | 7 | 7 |
| 1987 | - | - | - | 5 | 5 |
| 1986 | - | - | - | 5 | 5 |
| 1985 | - | - | - | 8 | 8 |
| 1984 | - | - | - | 3 | 3 |
| 1983 | - | - | - | 2 | 2 |
| 1982 | - | - | - | 2 | 2 |
| 1981 | - | - | - | 1 | 1 |
| 1980 | - | - | - | 3 | 3 |
| 1979 | - | - | - | 1 | 1 |
| 1968 | - | - | - | 12 | 12 |
| Total | 85 | 903 | 17 | 9808 |