DE ROSNAY, M. D. (2008) Check Your Data Freedom: A Taxonomy to Assess Life Science Database Openness. Nature Precedings. doi:10.1038/npre.2008.2083.1
There is an interesting discussion of the issues affecting accessibility (like "no policy visible on the web site"), and the article includes this good set of questions any data curators should ask themselves:
"A. Check your database technical accessibilityAll good questions to ask, although in some fields the right answer (for some datasets) to questions such as B.4 should be "No" (ethical considerations, for example, might dictate otherwise).
A.1. Do you provide a link to download the whole database?
A.2. Is the dataset available in at least one standard format?
A.3. Do you provide comments and annotations fields allowing users to understand the data?
B. Check your database legal accessibility
B.2. Is the policy clearly indicated on your website?
B.3. Are the terms short and easy to understand by non-lawyers?
B.4. Does the policy authorize redistribution, reuse and modification without restrictions or contractual requirements on the user or the usage?
B.5. Is the attribution requirement at most as strong as the acknowledgment norms of your scientific community?"