# Probability Question

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## 2 Replies - 1337 Views - Last Post: 22 November 2009 - 10:29 PM

### #1 tshauck

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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

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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.

Quote

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.

### #3 Momerath

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## Re: Probability Question

Posted 22 November 2009 - 10:29 PM

tshauck, 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