2 Replies - 858 Views - Last Post: 22 November 2009 - 10:29 PM

#1 tshauck  Icon User is offline

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Probability Question

Posted 22 November 2009 - 04:03 PM

Hi,

I have a question regarding probability. I know most people here enjoy math and not only computers - seeing as they go hand in hand. And I wanted to know if I was correct in my guess.

Say you have two independent events. Event 1 and Event 2. The chance that Event 1 occurs is .03 and the chance for Event 2 is .35. My assumption is that the likely hood of them both occurring is .03*.35. Where I'm getting stumped how often would they occur per year, assuming each event will happen once per day. Would it be .03*.35*365?

Thanks

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Replies To: Probability Question

#2 Oler1s  Icon User is offline

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Re: Probability Question

Posted 22 November 2009 - 08:42 PM

Yes, but only if you were to simply ask, what is the expected number of days with joint events.

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assuming each event will happen once per day
This is where things get funky. Each event doesn't happen once per day. Right? There's a probability of the event happening.
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#3 Momerath  Icon User is offline

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Re: Probability Question

Posted 22 November 2009 - 10:29 PM

View Posttshauck, on 22 Nov, 2009 - 03:03 PM, said:

Would it be .03*.35*365?

No, as that would result in a value greater than 1.

To figure out the chance of something happening at least once over a specified duration, you need to come at it backwards.

The chance that both events will happen in a day = 0.03 * 0.35 = 0.0105
The chance both events don't happen in a day = 1 - 0.0105 = 0.9895
The chance that both events don't happen in a day, for multiple days = 0.9895^# of days = 0.9895^365 = 0.021221
The chance that both events will happen in a day, at least once, in a year = 1 - 0.021221 = 0.978779
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