Sites That do not Pass on PR

[Boston30]

I've heard that some sites, which themsleves have high PR, do not pass on any PR to sites they link to. Not because of anything the site owner does, but because Google blocks the site from passing on PR. An example I heard of was a PR10 statistics site.

Does Google prevent some sites from passing on PR in this way (I'm referring to sites that have high PR themselves, not sites that are banned or anything)?

If this does happen how can I identify such a site when buying text links?

Thanks!

[xeno]

Links that have rel="nofollow" don't pass PR. It's possible that sites that aren't indexed do not pass PR as well, eg, sites that have PR, but not showing up on Google (even when you type in the sites url in the Google search bar).

[Sikky]

Thanks xeno I was not aware that links which have ref="nofollow" didnt pass. Will have to update my site.