"Graham also released some metrics on Y Combinator: it has 564 startups total. Of the 285 that have valuations, the total valuation of the companies is $11.6 billion."
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Obviously the average of $46 million doesn't provide a clear picture at all.
[1] - http://www.forbes.com/sites/quora/2013/02/07/why-is-dropbox-...
[2] - http://www.bloomberg.com/news/2012-10-19/airbnb-said-to-be-r...
By eliminating some of the biggest outliers, you're reducing the standard deviation, thus making the average metric more representative.
You'd also have to disregard the companies with very low or no value, but it seems this was already done in PG's original statement: "Of the 285 _that have_ valuations..."