Just wondered - how many of the following came from those 3 accounts?
•31.5% of tweets dealt with IGB, RFH, or total charity fundraising--that is, projects Cook knew about already.
•33.5% of tweets were other news items, votes in Faxo polls, casual mentions of Cook in other conversations, fans circulating photos and videos, and similar--information he already knew or had no pressing need for.

And how many of the retweets were retweets of those 3 accounts?

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I'm not sure if I agree with your methodology. If the purpose is to determine to what extent David's timeline is cluttered up by unnecessary tweets which mask the stuff he would be interested in, you should have taken a more representative sample. We don't know when and how often he checks his @ replies and mentions, but for example, I assume he would be likely to check it out when he has just tweeted, and probably when he knows he's likely to be a hot topic of conversations.