gpsbrar
Forum Elite
Topics: 34
Posts: 278
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Hi, I am stuck with calculating percentile from the standard deviations. All I know 1 standard deviation is 68% = 84 percentile (Basically answer for most of the Questions in the Qbank). But what if it is 2 standard deviations which is 95.5% and 3 standard deviations which is 99.7%. There is a question related to this concept for 1 standard deviation in Kaplan Book. Page 20. Q 2 in 2004 edition book. Ans is 84. Calculation is based 50 + (68/2) = 84. I could not figure out from where did the 50 come. Okay if this is correct them will be the same for 2 and 3 standard deviations respectively [2 dev.=(95.5/2)+50] and [3 dev.=(97.7/2)+50]. Please help. This is a sure shot question in the exam as we all know. They love to test statistics and standard deviation concept. Thank you for your time.
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| vallia
Forum Guru
Topics: 98
Posts: 889
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hey gpsbrar, I located the q. It is...In a normal distribution curve, what percent of cases are above 1s below the mean?
the 50 come from 50% below X(mean) and 50% above mean. And the calculation for percent of cases above -1s is 50+ 68/2=84. Or adding 34+34+13,5+2,4+0,15= 84
for calculating percents above -2s the formula will be 50+ 68/2+ (95,5-68)/2= 50+34+13,5=97,5.
Actually I don't know the formula but I guess this is it. Anyway I don't think we'll get such calculation. just remeber 1s---percentile 84 and 2s---percentile 97,5
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| study_ing
Forum Fanatic
Topics: 180
Posts: 2,511
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here unfortumately is no understanding which can help us. BY definition a Normal distribution has the given characteristics. if it didnt, it wudnt be called a normal distribution.
Do not try to learn the figures. always draw the figure out and insert the relevant numbers. all u have to learn is 3
those corresponding to +/- 1 sd, =/- 2 sd and 3 sd. i'll try finding an appropriate image (coz im just used to drawing it)
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| study_ing
Forum Fanatic
Topics: 180
Posts: 2,511
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fig 6.3 here http://www.westgard.com/images/ls36f4.gif
draw something out for urself something similar.and filll in values .remeber the central line means 50%
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