NDure Posted November 21, 2011 Share Posted November 21, 2011 The random variable X is normally distributed. The values 650 and 850 are the 60th and 90th percentiles of the distribution of X, respectively. A) The value at the 75th percentile of the distribution of X B) 750 Quote Link to comment Share on other sites More sharing options...
shridhar22 Posted November 21, 2011 Share Posted November 21, 2011 The answer is 750, col b i would like people to give an explanation though... its related to area under the curve. Quote Link to comment Share on other sites More sharing options...
nocturnal Posted November 21, 2011 Share Posted November 21, 2011 shridhar how did you reach the conclusion as B ?? Any hint would help us all solve the problem.. Quote Link to comment Share on other sites More sharing options...
caa5042 Posted November 21, 2011 Share Posted November 21, 2011 is it 650 + (850-650)*7.5%= 675 which is less than 750? I'm trying to get the hang of these normal distribution problems can someone offer an explanation.. Quote Link to comment Share on other sites More sharing options...
ReasonGRE Posted November 22, 2011 Share Posted November 22, 2011 First, let me say that on my exam I didn't see a whole lot of this new statistics stuff that they are suggesting will be part of the new GRE (random variables, normal distribution, interquartile range, etc.). I personally didn't see any of the aforementioned topics on my exam, and I got a 170Q, but obviously that doesn't mean you won't see it. You need to look at a bell curve to understand this. Since these two points are both to the right of the middle, you will notice that the curve thins out the further you go to the right. Now, 30% of the data pool is between the 60th and 90th percentiles. So you might expect 15% to be between 650 and 750 and 15% between 750 and 850. But since visually you can see that a greater percentage of the data is in the area closer to the 60th percentile, then you might imagine that something more like 20% of the data points might be in the area between 650 and 750 and maybe 10% would be between 750 and 850. These are just rough approximations, but the point is that if this is true, 750 would more likely correspond to something like 80th percentile and so 75th percentile would be below 750. Hope that helps explain it... Quote Link to comment Share on other sites More sharing options...
shridhar22 Posted November 22, 2011 Share Posted November 22, 2011 This can be understood by analysing area under the curve. ReasonGRE has tried to explain this.... but this question is not that easy to explain in posts. Even i had some confusion while trying it, good explanation can be found in "Cracking the New GRE 201" princeton review Quote Link to comment Share on other sites More sharing options...
caa5042 Posted November 22, 2011 Share Posted November 22, 2011 http://desmond.imageshack.us/Himg810/scaled.php?server=810&filename=stdj.png&res=medium Quote Link to comment Share on other sites More sharing options...
nocturnal Posted November 22, 2011 Share Posted November 22, 2011 Thanks for the chart caa, and thanks for the explanation everyone, but there is a little doubt .. what if instead of 75 percentile we had been asked for 85 percentile in column 1...?? Now what do we do and one basic question how come the value at 80 percentile is approximated to be 750...?? sorry, for such stupid questions, but i am a little weak on this topic.. Quote Link to comment Share on other sites More sharing options...
ReasonGRE Posted November 22, 2011 Share Posted November 22, 2011 To answer your questions, first, the question could not be asked with anything but 75%. You are not expected to know or be able to figure out exactly what the percntile would be for 750. This is outside the scope of the GRE. All they are looking for you to understand is that although 750 is half way between 650 and 850, more of the data points are in the half that is closer to 650, so the 75th percentile will be below 750. I don’t know if 750 would correspond to 80th percentile…as I indicated in my post, that was just an approximation….just a way to show that 75th percentile would fall below 750 since more that half of that 30% that falls between 650 and 850 is in the half closer to 650. Hope that helps! Quote Link to comment Share on other sites More sharing options...
steffin Posted December 2, 2011 Share Posted December 2, 2011 hope this helps! Quote Link to comment Share on other sites More sharing options...
harryblues18 Posted January 4, 2012 Share Posted January 4, 2012 First, let me say that on my exam I didn't see a whole lot of this new statistics stuff that they are suggesting will be part of the new GRE (random variables, normal distribution, interquartile range, etc.). I personally didn't see any of the aforementioned topics on my exam, and I got a 170Q, but obviously that doesn't mean you won't see it. You need to look at a bell curve to understand this. Since these two points are both to the right of the middle, you will notice that the curve thins out the further you go to the right. Now, 30% of the data pool is between the 60th and 90th percentiles. So you might expect 15% to be between 650 and 750 and 15% between 750 and 850. But since visually you can see that a greater percentage of the data is in the area closer to the 60th percentile, then you might imagine that something more like 20% of the data points might be in the area between 650 and 750 and maybe 10% would be between 750 and 850. These are just rough approximations, but the point is that if this is true, 750 would more likely correspond to something like 80th percentile and so 75th percentile would be below 750. Hope that helps explain it... can u gimme your mail id ? that would be helpful for me so that i can get some information of GRE materials . thank you Quote Link to comment Share on other sites More sharing options...
lomtepallavi Posted November 7, 2013 Share Posted November 7, 2013 The random variable X is normally distributed. The values 650 and 850 are the 60th and 90th percentiles of the distribution of X, respectively. A) The value at the 75th percentile of the distribution of X B) 750 Solution: since here difference of percentile is(90-60)= 30 percentile for point (850-650)=200. 30 percentile for 200 points. we need to get points score for 75 percentile(15 more percentile points then 60 percentile points) so 15 percentile=(200/30)*15=100 points so points on 75 percentile=650+100=750 THIS VALUE IS THE MAXIMUM VALUE AND IT CAN ONLY BE CONSIDERED IF GRAPH IS EQUAL AND NOT DECREASING. BUT HERE IN QUESTION IT IS MENTIONED THAT RANDOM VARIABLES ARE NORMALLY DISTRIBUTED SO THE GRAPH WOULD BE DECREASING IN A BELL SHAPE SO THE VALUE OF 75 PERCENTILE WOULD DEFINITELY BE LESS THEN 750 FOR SURE. Answer: Quantity B is greater :eager: Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.