When ideas that are first published in blogs have effects on the world outside the blogosphere, how strong is the correlation between the popularity of the blog where the idea first appears and the magnitude of the effect? How often does an idea that starts out in a less-popular blog become influential after it's linked to by a more-popular blog?

Funny you should ask ... the start of that comment, which I cut for stand-alone, was responding to someone who asked a very similar question:
This is what I'd written:

Now, regarding:

"What happens when the exact same post gets published by a "tail blogger"
and an A-list blogger? Who gets credit with the idea? and is it fair?

This is easy - to a very good approximation, if it's published by a
tail blogger, it's ignored. If it's published by an A-list blogger,
it gets echoed-chambered around as much as any item. I mean, why is
this even an issue? Look at it this way:

"What happens when the exact same post gets published to 100 people
and to 10,000 people? Who gets credit with the idea? and is it fair?"

Whether you think it's fair or not is an exercise in moral
philosophy. What happens is an exercise in mathematics.

If the blogosphere is just an echo chamber, then being an A-list blogger is as (in)significant, in the grand scheme of things, as being the most popular person in junior high school. If 99.9% of blog postings have no ultimate effect on the world outside the blogosphere -- the electronic equivalent of water-cooler chatter -- then I'm interested in the 0.1% of postings that do have an effect.

I also want to separate "can non-A-list-bloggers have an effect?" from "can the contributions of non-A-list-bloggers be properly attributed?" If I posted some original advice for my favorite political candidate on my blog, and a more popular blogger linked to it, and the candidate picked it up and used it, I'd be kvelling about having Made A Difference, even if most of the blogosphere had no idea where the candidate got the idea from.

The blogosphere is not 100.00% an echo chamber - just 99.9% 1/2 :-).
The 0.1% is of course the A-list and proportionally very few others.

Again, it's very much exactly like the larger mediamass. Most journalists merely echo what other journalists say, or wire services. Only a tiny number of journalists reliably have the power to make something a "Story". And of course we don't even have to ask whether something which starts out in East Nowhere has less of a chance of having an impact, than an item which starts in e.g. the New York Times

Non-A-list-bloggers basically only have an effect by sending in tips to powerful journalists, I mean A-list-bloggers. Otherwise, as a physical constraint, essentially nobody hears them. I've actually tracked this with some of my censorware reports, the numbers were discouraging.

By the way, take a look at the recent posting on Many 2 Many, especially propagation

Hmm. As you know, echoes within the blogosphere sometimes make it outside. I'm one of the few (apparently) who reads your stuff here--but I also use it elsewhere: In a zine with a (surprisingly) growing readership, which I'd estimate at 3,000 or so (given the number of people who say it's printed, stapled, and circulated around libraries, and given >1,900 typical unique downloads per issue at this point)--and, in some cases, in columns that reach 64,000+ people in print and some small number online. The current "Crawford Files," for example, quotes you directly:
http://www.ala.org/ala/alonline/thecrawfordfiles/crawford2004/january2004starting.htm

(And note that my CIPA Special, which certainly was influenced by your work, has just south of 4,000 unique downloads to date.)

That's still not A-list by any means, but your influence does reach beyond your direct readership. But, as you say, you can no more eat that or sleep under it than I can eat or sleep on the proceeds from Cites & Insights. I've been writing long enough to assume that I can only quit my day job when I retire...