Andy_Langton - 3:10 pm on Feb 26, 2013 (gmt 0)
I think the point was more that if a user has a directory of pages that they would like to rename or restructure, how much of the current pagerank is going to be lost using redirects rather than linking to the new pages (with a big red banner) OR if they are planning on moving from one TLD to another, how much PR is lost in the move.
I don't think this is true. He's answering the theoretical question:
If there are two identical links to page 'A' with one going via a 301 and the other links directly, is there any greater loss of pagerank for the 301?
In your examples above there are other reasons for loss of PageRank and other ranking signals. For instance, where existing content has moved, Google will compare the old content to the new destination and does not necessarily pass value at all if that doesn't add up.
There are other factors involved, of course. Do 100% of social likes travel across a 301? Does 100% of historic clickthrough data?
It can be dangerous to apply a theoretical point to a real world situation.
If a 301 redirect leaks PR, I would expect canonical to as well.
In a mathematical sense, yes. But in a real world sense canonical is less effective, since there are two URLs to evaluate. I've seen plenty of cases where canonical does nothing because of simple differences that occur between two Google visits to the two URLs (e.g. the news articles linked in a sidebar updated).
This is why (in real world scenarios) direct links still beat a 301, which still beats a canonical attribute.