| Year | value and error |
| 1989 [10] | 0.11 ± 0.01 |
| 1992 [11] | 0.1134 ± 0.0035 |
| 1995 [12] | 0.118 ± 0.003 |
| 1997 [13] | 0.118 ± 0.003 |
| 2000 [14] | 0.1184 ± 0.0031 |
| 2006 [15] | 0.1189 ± 0.0010 |
| 2008 [18] | 0.1198 ± 0.0032 |
| 2009 [6] | 0.1171 ± 0.0024 |
| 2012 [7] | 0.1173 ± 0.0011 |
| 2013 [16] | 0.1185 ± 0.0006 |