biostat.... Im having trouble... explain please? - zen786

[ArchivalUser]

15. A particular association determines membership on the basis of members' IQ scores.
Only those persons who have documented IQ scores at least 2 standard deviations above the mean on the Wechsler Adult Intelligence Scale (WAIS) are eligible for admission. Of a group of 200 people randomly selected from the population at large, how many would be eligible for membership to this society?
A. 1
B. 2
C. 3
D. 4
E. 5

can someone with a thorough explanation help??

the answer is E....

[ArchivalUser]

1)25% of the population have IQ less than 69
2)50% have IQ between 90 to 109 ie average
3)25% have IQ greater than 130

the study group in the question have superior IQ which is 25% of 200

ans=5

[ArchivalUser]

source;dr daugherty

[ArchivalUser]

You'll need the "bell curve of standard deviations of mean in a normal distribution" for this.Its on page 20 in my behavioral sciences kaplan book.!!...find this curve in your book first.
Now the question says only those who are ABOVE 2 S.D's above the mean are eligible...and we need to find out that number.Look at the curve carefully...the area to the left of +2s is all the ppl who are below 2 S.D's...the area to the right of +2s is all the ppl above 2 S.D's and thats what we need to find out...acc to the curve it would be 2.4% + 0.15%...approx 2.5%....and 2.5% of 200 is 5..

You'll need to memorize all those percentages given in the curve...we wont be provided with that curve in the examination

[ArchivalUser]

thank you so much @neilu78 and @harp

neilu i understand the +1,+2 and etc thats standard deviation... the above 68%/95 %/99% = the percentile i dont get the middle row numbers 34/13.5/2.4 can u shed a light on it please? thanks once again!!!

[ArchivalUser]

The top row percentiles are nothin but what you get when you add up these middle row ones...like if u are asked what percentile fall below 1s below the mean...i.e 1 S.D below the mean...look on the left side of the curve...ull need to add up all those middle row percentiles which fall below 1s(i.e to the left of 1s)...so it'll be 0.15%+2.4%+13.5%....its easy once u understand the concept...try solving the questions given below the curve...

[ArchivalUser]

68% is WITHIN 1(one) s.d...u get that by adding the 34% to the right and left of the mean...if they ask how many fall ABOVE the mean but below 1 s.d what'll u do?...ull hav to look at those middle row numbers right...it'll be 34% then.Those middle row percentiles are what u need to remember...

[ArchivalUser]

They are looking for people who are two standard deviations above the mean.

1 standard deviation away from the mean would include 68% of the group
2 standard deviation away from the mean would include 95% of the group
3 standard deviation away from the mean would include 99.7% of the group

[http://www.squarecirclez.com/blog/wp-con...-curve.gif] - In the following image, the population that we're focusing on the dark green shaded region on the right side.

(1) 95% of the population would include all but the green so that is 5%.
(2) BUT we are focusing on the population that is 2SD that is ABOVE the mean - so that would be half of the mean --> 2.5%
(3) The graph is based on a population of a 100, since this group has 200 people in it, we need to multiply it by 2 --> so final answer is 5.

Hope that helped!

[ArchivalUser]

Nice explanations


only @zen what would be the answer for this one.

A particular association determines membership on the basis of members' IQ scores.
Only those persons who have documented IQ scores at least 1 standard deviations above the mean on the Wechsler Adult Intelligence Scale (WAIS) are eligible for admission. Of a group of 200 people randomly selected from the population at large, how many would be eligible for membership to this society?