Statistics Prep (Guide 5 pg. 91)

[lab1219]

Hello!

I'm reviewing statistics strategy and have a question on book 5, pg. 91. I remember in class my GRE instructor suggested memorizing the exact percentiles in a normal distribution. The book doesn't exactly say this and the book doesn't give the exact percentiles. Do you recommend memorizing the percentiles? And if so, how? Or, specifically, can you please tell me the percentiles to memorize?

Thanks!

Lauren

[tommywallach]

Hey Lauren,

Yes, they are worth memorizing, though usually just having them as a ballpark is fine:

34, 14, 2

or 34, 14.5, 1.5 if you want to be even more exact. That's 1SD, 2SD, and 3SD in either direction around the mean.

Hope that helps!

-t

[amr5416]

In regards to this question, I memorized the percentiles as shown, which indicates that about 96% of the data will fall within two standard deviations of the mean.

I have a question about Problem 16 of Hard Word Problems Practice Sets on Page 185 of Strategy Guide 5. Since Joe's score was 2 standard deviations above the mean, I assumed he was in the 96%, like previous explanations said. However, the answer is 98%. If I use 96, answer choice A is still valid, but I don't know if my logic is correct. Can you please explain the percentile equation, [50%*(2/3)...] and what to plug in where, to get the percentiles for each standard deviation above or below the mean?

[tommywallach]

Hey Amr,

It just needs to be above 95% (I wrote it wrong up above, it should be 2.5% on each side, so that means 2SDs is 95th percentile or above). So anything above 95th percentile works.

-t