anonymous
  • anonymous
Applicants to a college psychology department have normally distributed GRE (Graduate Record Exam) scores with a mean of 544 and a standard deviation of 103. What is the GRE score at the 77th percentile? Round answer to the nearest whole number. A. 620 B.505 C.468 D.1 E.-1 not really sure how to do this problem, i did it two different ways and got two different answers and both those answers weren't a choice. Someone please help me walk through the steps if you know the answer :)
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katieb
  • katieb
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phi
  • phi
The first step is use a table to find how many standard deviations (above the mean) correspond to 0.77 Do you know how to do that ?
phi
  • phi
As you know, the area under the bell curve from the "left tail" up to the mean is 1/2 of the total area. In other words, 0.5 of the area is below 544
phi
  • phi
77% or 0.77 means we want the score where 0.77 of the total area is below that score. |dw:1444743374539:dw| common sense says that this score must be bigger than the mean 544 so we can answer the question by process of elimination: only 1 of your choices works.

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anonymous
  • anonymous
oh so it would have to be 620 then?
phi
  • phi
yes. 620 If we looked in a table, we would find 0.739 standard deviations above the mean is what we want. The std dev for this problem is 103, so we want to be 103*0.739= 76.1 points above the mean. so we want a score of 544+76= 620
phi
  • phi
Here is a useful tool http://davidmlane.com/hyperstat/z_table.html
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anonymous
  • anonymous
oh okay thank you, yeah the first time i looked at a z-score table i got the value wrong for .77 thank you!
phi
  • phi
If they want you to learn how to use a table (which may be confusing) , you should practice until it makes sense.

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