# Thread: Semi Interquartile Range of a population

1. ## Semi Interquartile Range of a population

The population mean is 500, the standard deviation is 100. How would I go about finding the semi interquartile range for this? Would I use the unit normal table to look up proportions of .7500 and .2500? I'm not sure if I should look in the body or the tail for each one. Please Help!

Thanks.

2. ## Re: Semi Interquartile Range of a population

Originally Posted by OreoKid
The population mean is 500, the standard deviation is 100. How would I go about finding the semi interquartile range for this? Would I use the unit normal table to look up proportions of .7500 and .2500? I'm not sure if I should look in the body or the tail for each one. Please Help!

Thanks.
The semi interquartile range seems to be half the difference of the first and third quartiles.

So you'd find out the values of the 0.25 and 0.75 on the CDF of the Normal distribution N(500,100)

I get z(0.25) = -0.6745, that should tell you what z(0.75) is and you can denormalize it in the usual way.

Then just take half the difference of the two.