1. ## nonstandard normal distributions

I am working a problem regarding IQ scores and am having trouble with one of the problems. I've got it figured out how to work other problems but can't figure out how this problem is figured out.

I've got to find P10, which is the IQ score separating the bottom 10% from the top 90%. Mean of 100 and standard deviation of 15.

I got the answer from the back of the book but can't figure out how to get it.

2. ## I.Q. test

Originally Posted by jlpickett
I am working a problem regarding IQ scores and am having trouble with one of the problems. I've got it figured out how to work other problems but can't figure out how this problem is figured out.

I've got to find P10, which is the IQ score separating the bottom 10% from the top 90%. Mean of 100 and standard deviation of 15.

I got the answer from the back of the book but can't figure out how to get it.

Here is how I did it, let's see if I am right:

First, I made a Standard Graph showing that we want the bottom 10% shading to the left.

Then, let (mean) u=100, (Standard Diviation) o=15 and we find for P10 (which I am calling x)

Next, if you are working in a Stats book, it should have some sort of Z-Score table which you can look up what the Zscore of 10% is, in this case -1.28, then plug it into the formula: x=u+(z*o)

We get x=100+(-1.28*15); x=80.8

Therefore; x=81 (rounded) I.Q. score separates the bottom 10% from the top 90%.

Someone correct me if I am wrong.

3. Originally Posted by DryMouse
Here is how I did it, let's see if I am right:

First, I made a Standard Graph showing that we want the bottom 10% shading to the left.

Then, let (mean) u=100, (Standard Diviation) o=15 and we find for P10 (which I am calling x)

Next, if you are working in a Stats book, it should have some sort of Z-Score table which you can look up what the Zscore of 10% is, in this case -1.28, then plug it into the formula: x=u+(z*o)

We get x=100+(-1.28*15); x=80.8

Therefore; x=81 (rounded) I.Q. score separates the bottom 10% from the top 90%.

Someone correct me if I am wrong.

You are correct.

4. ## i think i understand

Ok, I think I understand. 80.8 is the correct answer according to the textbook. I just couldn't figure out how to find the z score or the formula to get the answer. Thanks so much for your help and quick response. Now let's see how I do applying the formula to the next problems.

5. The easiest way I know is if your text book has a Positive and Negative Z-Scores table in an Appendix somewhere. Then, all you have to do is look up .1000 (for 10&#37 in the body of the table and find the closest corresponding Z-Score. In this instance, since we are looking for data to the left of the graph, it will be negative on the table. So looking at the Negative table, I found .1003 in the body, which was real close to our target of .1000 and followed the line to the far left and found -1.2 and then followed the line to the top and found .08, put them together and you get the Z-Score of -1.28.

I hope that makes sense.

Also a quick note, x=u+(z*o) is the formula when you have the z-score, but looking for x, if you have x and looking for the z-score, then you solve for z and get z=(x-u)/o

6. ## thanks

makes complete sense. THANK YOU

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