Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.
Sources of items in the pool.
| Number of items | Sources |
|---|---|
| 37 | Hsu (2005) |
| 19 | Dang (2012), Yang (2007) |
| 18 | Chan et al. (2002) |
| 08 | Le (2013) |
| 07 | Cotterall (1999) |
| 07 | Swatevacharkul (2009) |
| 03 | Ming and Alias (2007) |
| 02 | Dixon (2011) |
| 02 | Cotterall (1995) |
| 04 | Researchers in this study |
