. 2004 Nov 8;4:25. doi: 10.1186/1471-2288-4-25
Copyright © 2004 O'Cathain and Thomas; licensee BioMed Central Ltd.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Figure 1.
Different types of open questions in surveys