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 case-control study  



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  #1

A case-control study is conducted to assess the
risk for
intussusception in infants under the age of 1 year who
receive the rotavirus
vaccine. The medical records of all those who
received the vaccine and
those who did not receive the vaccine over a 6-month
period are
reviewed. Results show 125 cases per 100,000
infant-years for infants who
received the vaccine compared to 45 cases per 100,000
infant-years for
infants who did not receive the vaccine. The
investigators conclude that
the relative risk for intussusception is 1.9 times
greater in infants
who receive the rotavirus vaccine (95% confidence
interval of 0.5–7.7 and
p=0.39). Which of the following is the most accurate
interpretation of
these results?

A
) The results do not show an association between
rotavirus vaccine
and intussusception, but they may be related

B
) The results show sufficient statistical power to
identify an
association between rotavirus vaccine and
intussusception

C
) Rotavirus vaccine is associated with a 39% risk
for
intussusception

D
) Rotavirus vaccine causes intussusception in 1.9%
of infants

E
) Rotavirus vaccine prevents 80 cases of
intussusception per 100,000
infant-years





  #2

I'll go with A) The results do not show an association between rotavirus vaccine and intussusception, but they may be related.

Reading the stem indicates that there's an excess of 80 cases of intussusception in the vaccinated kids using the number of people in the denominator. That might clue you into blaming the Rotavirus vaccine for the excess of cases. But this is a case-control study which usually is retrospective, so another variable might still be responsible for that association (the study hasn't been randomized). But the strength of that possible association needs statistical confirmation, and that comes from the RR (Relative Risk) of 1.9. Notice that its Confidence interval at 95% ranges all the way from 0.5 to 7.7. In other words, it goes from a protective range (0.5; rotavirus protects from intussusception) to harming range (7.7; rotavirus goes with intussusception). If the RR is 1.0 means no risk. If <1.0 is protective; if >1.0 harming but in order to be one of them its corresponding CI-95% has to remain either in the protective range (say 0.7-0.9) or in the harming range (say 3.3-7.7). When the RR spans all the way from <1.0 to >1.0 (protective to harming) it suggest that crosses 1.0 or no risk. This is further strenghtened by the statistical significance given here, which is larger than 5% (p>0.05). In this case, the p value was 0.39 or 39% which means that RANDOMNESS explains the 39% of the differences between the number of cases of Intussusception vs no-Intussusception. Too high level of randomness!!!. Maximum tolerable is 5% (sometimes less if the sample is too huge, in that case Bonferroni correction needs to be applied).

So what's the Confidence Interval at 95%? In this case they're talking about the CI for RR. It means that if you repeat the experiment 100 times; in 95 of those times the RR would range from 0.5 up to 7.7. In other words, from protective passing through 'no risk =1.0" and going up to a lot of risk (7.7). If it's going to protect or is going to harm it has to stay on only one side. You can not be good or bad at the same time.

Hopefully I got it right





  #3

good explanation but can u explain why b is wrong?
i remember if u increase the sample size u increase the power ?


  #4

mn2_200 wrote:
good explanation but can u explain why b is wrong?
i remember if u increase the sample size u increase the power ?


The power of the study is OK but the study's design is flawed. It's a case control study and there's so much you can conclude from that. A better study would be a prospective cohort study and the best of course would be a clinical trial randomized and placebo controled, double blind.


  #5

Agree with Xeno (A)! im not very good in stat but I remember those few things; if CI contains #1 means NO statistical difference which makes a contradiction with the study results.





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