If you could know only one statistic about an alien civilization

Adam asks:

If you were offered a true statistic about an alien civilization, but only one, what would it be?

How about the real rate of return on capital?  The risk premium?  The percentage of the population which dies in war each year?  Those are what come to mind right away.  What else?  Ideally you might want a cognitive measure, but their performance on human IQ tests probably would not be useful information.  How about "what percentage of our knowledge of mathematics do they also have?"  Furthermore, I would not assume they "look like the aliens you see on TV" and would consider a biological statistic (which one?) which expressed what kind of life forms they would count as.

What would you choose?


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