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#1
02-03-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 646 Threads: 52 Thanked 762 Times in 256 Posts Reputation: 773
confidence interval question

what happens to the confidence interval if you increase the number of test subjects

eg.. right now CI== 5-45
and N= 50 ... if you increase N to 1000

CI=?

increase
decrease
increase or decrease (unpredictable)

#2
02-03-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 851 Threads: 88 Thanked 710 Times in 380 Posts Reputation: 720

CI= range from [mean+Z(sd/square root N)] to [mean -Z(sd/sq root N)]

i dont know how to write square root ,i hope u understand .

for ex if CI is 95% then Z is 1.96 ,sd =standard deviation and N is sample size

so if u take a bigger sample the CI will shrink ,i.e CI will become smaller.
because when N increases the range around mean will decrease(from the formula)

For eg -just to make u understand.imagine a class of 10 students,okay.we find mean score of step 1 score in that class is 240 ,with a 95% CI of 200-250.

now take a class of 200 students from another med school and found meaan score was 225 with a 95% CI of 215-230.

95% CI is 95% chance of getting a value ,when u randomly pick a subject in that study group.
 The above post was thanked by: Carolina_USMLE (12-23-2013), docusmle9 (02-03-2012), mbbs2010 (02-03-2012), Thrax_usmle (12-07-2012)
#3
02-03-2012
 USMLE Forums Guru Steps History: 1+CK+CS+3 Posts: 391 Threads: 98 Thanked 164 Times in 101 Posts Reputation: 174

i dont know i am making a guess from formula as stated in prev explanation

now as mean= total sum/sample size

so no mater what sample size increases total sum will be more i gues

so the mean will increase.......as mean increases the CI will increase by pure logic that has more weightage in influencing CI than SQUARE ROOT OF NUMBER.

so CI will increase. which i think means will be more sample in the CI than outside it.

is this same what you were saying?step_enhancer?
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 The above post was thanked by: mbbs2010 (02-03-2012)

#4
02-03-2012
 USMLE Forums Addict Steps History: Step 1 Only Posts: 177 Threads: 33 Thanked 86 Times in 16 Posts

It wud increase
#5
02-03-2012
 USMLE Forums Master Steps History: Not yet Posts: 928 Threads: 17 Thanked 539 Times in 375 Posts Reputation: 549

It will decrease ...... this concept was tested in one of the NBME , step enhancer has explained it correctly ...
#6
12-13-2014
 USMLE Forums Newbie Steps History: Not yet Posts: 9 Threads: 5 Thanked 3 Times in 2 Posts Reputation: 13

it would increase, e.g looking for a freind in a room whats the confidence dah ull find him if u search 2 rooms dan if u search 4 rooms , the higher the number the higher the confidence interval

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