Thursday, October 08, 2009


Something that perplexes me....

I just came across this bit about asteroid Apophis and the chances of it impacting earth.

"NASA scientists have recalculated the path of a large asteroid known as Apophis and now say it has only a very slim chance of banging into Earth.. The Apophis asteroid is approximately the size of two-and-a-half football fields, and updated computational techniques and newly available data indicate the probability of an Earth encounter on April 13, 2036 for Apophis has dropped from one-in-45,000 to about four-in-a million, NASA stated."

So here's the bit that rubs me wrong: "four-in-a million". Why not say "one-in-250,000". If the authors are really stuck on the whole "four-in" thing then they should have said four-in-180,000 to start with.

I really find inconsistencies like this baffling, I wish I knew the author and could understand what they hoped to gain by reporting the number in this way.


I'm with you on this one. My guess is they just wanted to use 'a million' to try to show high contrast to previous numbers.

I'd also argue that they've already blown their accountability by employing the 'football field' unit of measurement...
Well, I see it like this. To say "4 in a million", implies
it could happen 4 times in a million years which does
not mean every 250,000 years.
Reducing a ratio changes the meaning.
For example: If the ratio of boys to girls in a classroom is 20 to 15, you know there are 35 students
in the class. Reducing the ration implies there are
only 7 people in the class, which is not true.
I don't believe ratios should be treated as fractions.
Their use and meaning is different, or else why were
they invented?
@Herndon's Math House

That's an interesting take that I hadn't considered. However, considering the subject of the analysis hasn't changed I still fail to see the reason to change the scale between the two analysis.

I'd also argue that probability or chance isn't really a ratio. Unlike your class example where 20 to 15 implies a total of 35. one-in-45,000 or found-in-a-million do not means there are 45,001 chances or 4,000,004 chances that it might happen.

Also note 20-to-15 is applicable to a set of any multiple of 35. So a class of 70 could still have a ratio of 20-to-15.
GAH....can you tell I wasn't focusing when I posted that. Yes I typo'd "four-in-a-million" :(
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