Expert: Ahmed Salami - 2/4/2012

Question
You answered on 1/17 a question for pradeep that involved z values. I have the same question on my assignment but I am not sure where ther 1.175 value came from? I know it's from a table but I am not finding it. Which table? Please help. Until I figure out how to do that question myself I cannot continue on to take my exam and I'm short on time to finish my assignments for the week. I hope to hear from you soon.

Answer
Hi Kelly,
As i mentioned in the original solution there are different variations of the normal distribution table but they serve the same purpose in the end. I have attached two different tables to this solution for you to see what i'm talking about.
Now, usually we use a z-value to determine area (or probability) from the table but in this situation we have the area and we require the z-value so we work backwards. We look into the table for the area in that particular table and deduce the z-value from there.
Looking at the first table we see that it gives the area to the left of z. But we require the value of z such that the area to the right is 0.12 (as stated in the question) and so the corresponding area with which we can find z on the table would be 1 - 0.12 = 0.88
This is equivalent to a z-value of about 1.1 + 0.08 = 1.18
If we wanted to use the second table instead, we see that the area it gives is from 0 to z. For an area to the right of z equal to 0.12 the area we need in order to get z from this table would be 0.5 - 0.12 = 0.38
And again we see that it corresponds to a z-value of about 1.18, agreeing with the first table and the more accurate value of 1.175 (which i didnt get from tables but from computation).

I hope this helps. And you can always get back to me.

Regards