Question

Simple reliability/probability question

Asked by: dopyiii

Quick reliability/probability question for you:

You have a batch of light bulbs.  Each light bulb has  a mean wearout time of 10,000 hours and a standard deviation of 75 hours.

a) what is the probability of a light bulb failure before 9700 hours?
b) what is the probability of a light bulb failure after 10,500 hours?

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Asked On
2007-03-09 at 07:32:10ID22439143
Tags

probability

,

reliability

Topic

Math & Science

Participating Experts
2
Points
250
Comments
4

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Answers

 

by: grg99Posted on 2007-03-09 at 08:02:05ID: 18688098

9700 is four standard deviations below 10,000.   Use the formula, plugging in "4"

10,500 is 6.6666 standard deviations above.  Same formula.

 

by: dopyiiiPosted on 2007-03-09 at 08:17:09ID: 18688228

Okay, I got that.  I guess "the formula" is where I'm getting confused.  Which formula?

 

by: InteractiveMindPosted on 2007-03-09 at 08:23:59ID: 18688283

As you probably know:

To determine the number of standard deviations away something is, you use:
Z = (X - mu)/sigma
(where mu is the mean, sigma the standard dev.)

Then for (a), you use:
phi(z)

for (b):
1-phi(z)

 

by: dopyiiiPosted on 2007-03-09 at 08:36:38ID: 18688448

Got it.  Thanks!

20120131-EE-VQP-002

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