A study is performed to evaluate the incidence of peri-operative stroke during a carotid endarterectomy based on whether the surgeon uses a shunt during the procedure or not. The authors calculate the odds ratio of suffering from stroke if the surgeon uses a shunt is 0.84 (95% CI: 0.72 – 1.12) compared to when the surgeon does not use a shunt. What is the most appropriate interpretation of this finding? | |
A. Use of shunt decreases risk of stroke by 84% | |
B. Use of a shunt does not decrease or increase risk of stroke | |
C. Use of shunt does not decreases the risk of stroke | |
D. Use of shunt decreases risk of stroke with a 0.84 odds ratio | |
E. Use of shunt decreases risk of stroke by 16% |
Filed under: USMLE STEP III QUESTION BANK, USMLE Test Prep | Tagged: Archer Bio-statistics, USMLE Step 3 Bio-statistics |
D
B
B
B since the confidence interval includes the value 1
C
b
b, contains null value, not statistically significant
Would nt the answer depend on the null hypothesis? If the null hypothesis were a one way hypothesis saying that placing the shunt does not decrease the incidence of stroke, the p value being > 0.05 (since CI contains value 1), we would “not reject it” and thus we can say it does not decrease incidence of stroke.
However if the null hypothesis is 2 way saying there is no difference in incidence of stroke (in other words the incidence of stroke is the same for with and without shunt or there is no increase or decrease in the risk of stroke), the p value being > 0.05 we again do not reject the null hypothesis and can then conclude that there is no increase or decrease in the risk of stroke by placing a shunt.
Would really appreciate if someone with more biostat knowledge could explain if this thinking is correct or not and also how to get to the correct answer to this question
Thank you
B
B
Answer: B. Here is an explanation to the question.
The odds of stroke risk using the shunt is 84% (16% better than the procedure done without the shunt) with the TRUE population “CERTAIN” EFFECT AND BENEFITS are between 28% (100% – 72%) and (-12%)…{this is negative 12%…➡️ 100% – 112% = -12%} which is IMPOSSIBLE.
This is why the CI can NEVER be greater than 1.
Notice that I said here the word “CERTAIN” effect and benefits, this is because Confidence Interval ALWAYS represents the UNcertain effect and benefits, in this case they are between (72% – 112%) or (0.72 – 1.12).
In other words, let’s take this example for more clarification:
if the OR is 0.84 with 95% CI (0.3 – 0.8), how are you should you interpret this?
Answer:
The odds of stroke is 84% with the true population benefits (from the shunt) are between 70% and 20%.
I hope that I clarified and simplified the concept.
For the “P” value, it means:
What is the chance that your calculations of the CI were random and are not true values.
Obviously the smaller the chance of randomness, the better off you are.
P < 0.001 (1/1000) is much better than P < 0.01 (1/100).
P < 0.001 is statistically more significant
than p < 0.01
B.
CI contains 1, so the shunt does not increase or decrease the risk of stroke. If confidence interval was say, 1.5- 2.5 then shunt would increase the risk of stoke compared to not using shunt. If CI was like- 0.5-0.80 then shunt will be decreasing the risk of stroke.
why not ans. C; because when you say shunt does not decrease the risk of stroke, it could also mean it might increase the risk of stroke or shunt don’t affect the outcome. Where as answer B addresses this issue completely. so B is best choice.
Dr Red can you please explain what is the right answer and why? because it is very hard for me to understand what others are trying to explain.
Thanks