Standard deviation

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For a home emergency insurer the time taken from initial call for help to arrival of a tradesman is normally distributed with the parameters:

Mean no. of mins - 60
standard deviation - 10

Calculate the value of the probability that the time taken will be between 45 and 75??

I've got a figure of 82.2% but not sure its right!! My coursebook is a bit rubbish and implies the value of z can only be calculated at full z intervals, not half intervals, which is exactly what 45 and 75 is?

Thanks

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[+3] - JPB - 03/10/2010If the exercise is one of interpolation, however, and you were supposed to interpolate between Z=1 and Z=2 to get your result, then your answer is pretty close to the 81.85% that that method yields.

@PHIL5955,

I get 86.64%

Here's a better z table.
http://www.statsoft.com/textbook/distribution-tables/#z

You have 1.5 std deviations on either side of your mean. 1.5 std deviations = 43.32% of the area under the curve. Multiply by 2.

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@JPB,

Thank you

1 Reply

@PHIL5955,

If the exercise is one of interpolation, however, and you were supposed to interpolate between Z=1 and Z=2 to get your result, then your answer is pretty close to the 81.85% that that method yields.

1 Reply

@JPB,

Thanks for the comment, great help

1 Reply

@PHIL5955,

You're welcome.

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@JPB,

http://www.fourmilab.ch/rpkp/experiments/analysis/zCalc.html

this z score calculator shows plus and minus z 1.5 is 86.64% amazing coincidence?

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Standard deviation

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