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#1
07-01-2012
 USMLE Forums Scout Steps History: Step 1 Only Posts: 18 Threads: 4 Thanked 11 Times in 8 Posts Reputation: 21
Confidence Interval Question

Anybody interested in confidence intervals? Here is the question
-researchers wish to determine the efficacy of a new weight loss regimen. to do this , they compare the amount of weight lost by 40 patients on the regimen (Group A) with 40 people selected at random from a commercial weight loss program (Group B) and 30 volunteers who were trying to lose weight on their own (Group C). results are shown in Table 1.
Mean SE N
Group A 17.3 lb 3.4 40
Group B 12.7 lb 4.3 40
Group C 5.4 lb 5.3 30

Based on the results shown and using a criterion level of P less than 0.05, the researchers are most likely to conclude that
a)Group A lost more weight than Group B, and Group B lost more weight than Group C
b)Group A showed more weight loss than Group C, but not more than Group B
c)no answer is possible without the results of a statistical test being given
d)no significant differences were found among the three groups
 The above post was thanked by: ..sharma (07-01-2012), beka-CTS (07-01-2012)

#2
07-01-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,039 Threads: 189 Thanked 557 Times in 318 Posts Reputation: 567

a)Group A lost more weight than Group B, and Group B lost more weight than Group C
 The above post was thanked by: Marikella (07-01-2012)
#3
07-01-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 763 Threads: 77 Thanked 1,338 Times in 347 Posts Reputation: 1348

d)no significant differences were found among the three groups

CI for 95% and 99% for groups A,B & C overlaps each other. I guess they aren't significantly different.

Nice Question. We've kicked biostats ass
 The above post was thanked by: Marikella (07-01-2012)
#4
07-01-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,039 Threads: 189 Thanked 557 Times in 318 Posts Reputation: 567
?

Quote:
 Originally Posted by beka-CTS d)no significant differences were found among the three groups CI for 95% and 99% for groups A,B & C overlaps each other. I guess they aren't significantly different. Nice Question. We've kicked biostats ass
so SE here is a standard error of the mean? which means we don't divide by sqr of N? I thought it's simply SD and we need to divide it by N...
#5
07-01-2012
 USMLE Forums Scout Steps History: Step 1 Only Posts: 18 Threads: 4 Thanked 11 Times in 8 Posts Reputation: 21

Ok, the correct answer is D
this question needs several steps to come up with the correct answer
1) because p is less than 0.05 it tells you to use 95% confidence interval
2) with that 95% CI, Z score is 2
3) if you do the calculation for each group 17.3 (+-) 2x3.4
12.7 (+-)2x4.3 and 5.4 (+-)2x5.3 you will see that ALL the Confidence Intervals overlap

so there is no statistically significant difference between the groups!
 The above post was thanked by: ..sharma (07-01-2012), Zamalek (07-01-2012)
#6
07-01-2012
 USMLE Forums Scout Steps History: Step 1 Only Posts: 18 Threads: 4 Thanked 11 Times in 8 Posts Reputation: 21

yes casandra!
they have already given you standard error, so you don't need to divide
so the numbers of patients 40, 30 and 40 are extra, you don't need this information really..
 The above post was thanked by: amit chahar (07-14-2012)
#7
07-01-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,039 Threads: 189 Thanked 557 Times in 318 Posts Reputation: 567
:)

Quote:
 Originally Posted by Marikella yes casandra! they have already given you standard error, so you don't need to divide so the numbers of patients 40, 30 and 40 are extra, you don't need this information really..

ooh thanks ok, that makes sense. then without that extra dividing that I have done, the CIs overlap so there is no statistically significant difference D.

 Tags Biostatistics-Epidemiology, Step-1-Questions

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