| Study | Date of the Study |
|---|---|
| The first study | September 1997 |
| The second study | August 1999 |
| The third study | June 2001 |
| The fourth study | April 2002 |
| The fifth study | December 2005 |
| The sixth study | September 2009 |
| The seventh study | November 2013 |
| The eighth study | December 2014 |
| The last study | May 2017 |