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#1
01-26-2016
 USMLE Forums Newbie Steps History: Not yet Posts: 7 Threads: 3 Thanked 1 Time in 1 Post Reputation: 11
Hard Biostatistics Questions.

A 48-year old man is referred for evaluation of possible hypertension. On the basis of ten measurements, the patient's average diastolic blood pressure is 113mmHg, and the standard deviation is 8mmHg. If four rather than ten measurements of the mean blood pressure are made, which of the following is the expected impact on the size of the 95% confidence interval about the mean blood pressure?

A) Change, but the direction cannot be predicted
B) Decrease in width
C) Increase in width
D) Remain the same

As far as I know, and allegedly, the answer is C.

But I chose A because you never know what would happen to 4 newly measured values' standard deviation. I mean, it was 8mmHg when you measured 10 times, there is possibility it would be 1mmHg when measured 4 times with minor probability. I also understand that you should use σ(the population standard deviation), but you can use s(standard deviation within sample) instead where you can't find σ, which happens almost always in the field of inferential statistics including this situation.

So if this is the case, the size of the 95% confidence interval decrease even if (root 10) is larger than (root 4), because 8mmHg is far larger than 1mmHg.
And of course there is possibility that 95% confidence interval remains same contrary to choice A, but the chance is nearly zero because blood pressure is interval scale. If you take any 2 real number which is interval scale between 0 and 1 , the chance that you get same 2 number is zero.
These were the reasons why I thought A is answer.

Please tell me what's the right way to think.
 The above post was thanked by: chanduM (03-12-2016)

#2
01-26-2016
 USMLE Forums Newbie Steps History: Not yet Posts: 7 Threads: 3 Thanked 1 Time in 1 Post Reputation: 11
Second Hard Biostat Question

An 80-year-old woman is being evaluated for suspected temporal arteritis(TA). Her erythrocyte sedimentation rate(ESR) is 100mm/h. Treatment of TA involves the use of glucocorticoids, which could have serious adverse effects. The pretest probability for TA is 50% in this patient. In the evaluation of TA, ESR has a sensitivity of 99% and a specificity of 60%. Based on the results of the ESR testing in this patient, which of the following is the most appropriate next step in management?

A) Additional testing to confirm the diagnosis of TA
B) Corticosteroid therapy, since the diagnosis of TA has been established with 99% certainty
C) Elimination of TA from further diagnostic consideration
D) Repeat ESR; if again positive, corticosteroid therapy
E) Repeat ESR; if normal, additional testing to confirm the diagnosis of TA

My choice was D but A is allegedly correct answer.
I used Likelihood ratio and pretest probability assuming sensitivity is 1(which is almost the same as 99% and doesn't make much difference) to calculate posttest probability which was 5/7.(LR : 2.5, Pretest probability is 0.5 according to question stem). Below is the formula I used.
· P' = P0*LR/(1-P0+P0*LR), where P0 is the pre-test probability, P' is the post-test probability, and LR is the likelihood ratio. This formula can be calculated algebraically by combining the steps in the preceding description.(quoted from WIKI 'likelihood ratio')
as P' is equal to positive predictive value, PPV is alreay 5/7. If you do it once again like option D and assume it comes out positive again, then I can calculate 2nd P', which I call P''(double prime) and is 25/29=86% (you use 5/7 instead of 0.5 this time in the position of P0 in formula above). I believe this number is great enough to diagnose Temporal Arteritis and give steroids to prevent blindness with all 'serious adverse effects' because the benefit outweighs the harm here.

Help me!!!
#3
01-26-2016
 USMLE Forums Guru Steps History: Step 1 Only Posts: 305 Threads: 40 Thanked 103 Times in 64 Posts Reputation: 113

Well I understood the first question
Confidence interval= mean +/_ standard deviation
As u see the equation of SD the sample n comes in the denomitor
So as the same decreases the confidence interval width increases.......the range I mean

Eg if the mean is 100 and std dev was 10......so CI is now 90-110
But if the sample size decreases so that the SD is increased.....for eg to 20 then CI is now 80-120
So the range width increases

I'm not sure abt the second ques
Maybe it s because the specificity is low which actually rules in the disease? But I'm sure this is nt thr ryt explanation.....pathoma says we shud give steroids even if we suspect TCA.... And tat kind off and is there in the options to confuse me!! Lol!!
 The above post was thanked by: CHASER (01-27-2016)
#4
01-26-2016
 USMLE Forums Scout Steps History: 1 + CK Posts: 35 Threads: 1 Thanked 3 Times in 1 Post Reputation: 13

at the first question i had the same reasoning as you did...

At the second question I choose A but I cannot give you a logic pattern. I just thaught if I had a patient with onliy ESR of 100 would I treat for TA? no...60% it's just not enough for making a dagnosis and I don;t think it's high enough to start treatment
#5
01-27-2016
 USMLE Forums Scout Steps History: Not yet Posts: 84 Threads: 1 Thanked 49 Times in 35 Posts Reputation: 59

The answer for the second question is A in my opinion.

A positive ESR means that you can not rule out temporal arteritis (high sensitivity, meaning that if you take 100 patients with known TA, 99 of them are going to test positive on ESR) but since elevated ESR can be caused by a million things other than temporal arteritis you need some other more specific test to confirm it (biopsy?).

That would be an answer to a pure biostatistics question.

But I think I heard on pathoma that a suspicion of temporal arteritis is enough of a reason to start steroids even before you test for it since it can cause blindness (I do not know how correct that is). But B is incorrect since 99% sensitivity does not establish a diagnosis with 99% certainty. A 99% positive predictive value establishes a diagnosis with 99% certainty, not sensitivity.

C is not correct because you just tested positive on a highly sensitive test. Which by definition means you can not rule out a diagnosis (99% of people with known TA test positive on this test). A negative result on a sensitive test would eliminate the possibility of temporal arteritis (because only 1% of people with known TA test negative on this test).

D does not look right. I think the person who started this thread is overthinking the question. Those board style questions like to give a long stem with a crapload of unnecessary information to try and throw you off. Also, Step 1 will not require you to judge if 86% is a number high enough to diagnose TA. Doing the test again and getting a positive result does not change anything. The test still has low specificity. No matter how many times you do the test it will not become highly specific and it will never allow you to confirm a diagnosis. A second positive result on a test will tell you that based on this result you can not exclude the possibility of TA, the same thing that the first test result told you.

E does not make any sense. If ESR were normal that would rule out TA (again, only 1% of patients with known TA test negative on this test)

The fact that pretest probability is very high in this patient it increases PPV and decreases NPV. Which kind of messes with you, but still, you get about a 71% PPV and a 98% NPV with this pretest probability.
 The above post was thanked by: CHASER (01-27-2016), Kruno (03-11-2016)
#6
01-27-2016
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 83 Threads: 2 Thanked 39 Times in 30 Posts Reputation: 49

Quote:
 Originally Posted by CHASER An 80-year-old woman is being evaluated for suspected temporal arteritis(TA). Her erythrocyte sedimentation rate(ESR) is 100mm/h. Treatment of TA involves the use of glucocorticoids, which could have serious adverse effects. The pretest probability for TA is 50% in this patient. In the evaluation of TA, ESR has a sensitivity of 99% and a specificity of 60%. Based on the results of the ESR testing in this patient, which of the following is the most appropriate next step in management? A) Additional testing to confirm the diagnosis of TA B) Corticosteroid therapy, since the diagnosis of TA has been established with 99% certainty C) Elimination of TA from further diagnostic consideration D) Repeat ESR; if again positive, corticosteroid therapy E) Repeat ESR; if normal, additional testing to confirm the diagnosis of TA My choice was D but A is allegedly correct answer. I used Likelihood ratio and pretest probability assuming sensitivity is 1(which is almost the same as 99% and doesn't make much difference) to calculate posttest probability which was 5/7.(LR : 2.5, Pretest probability is 0.5 according to question stem). Below is the formula I used. · P' = P0*LR/(1-P0+P0*LR), where P0 is the pre-test probability, P' is the post-test probability, and LR is the likelihood ratio. This formula can be calculated algebraically by combining the steps in the preceding description.(quoted from WIKI 'likelihood ratio') as P' is equal to positive predictive value, PPV is alreay 5/7. If you do it once again like option D and assume it comes out positive again, then I can calculate 2nd P', which I call P''(double prime) and is 25/29=86% (you use 5/7 instead of 0.5 this time in the position of P0 in formula above). I believe this number is great enough to diagnose Temporal Arteritis and give steroids to prevent blindness with all 'serious adverse effects' because the benefit outweighs the harm here. Help me!!!
this question is tricky. the post above explains it quite well. so basically to sum it all up, ESR is a screening test which in this vignette was positive. most of the options above are trying to trick you. remember principle of biostats 101. once a screening test is positive (ESR here), you move onto the confirmatory (specific test) which is temporal artery biopsy for TA.
 The above post was thanked by: CHASER (01-27-2016), zak09 (03-12-2016)
#7
01-27-2016
 USMLE Forums Newbie Steps History: Not yet Posts: 7 Threads: 3 Thanked 1 Time in 1 Post Reputation: 11
Thanks all guys.

I am reminded of two principles.

1.Back to basics.

Thanks everybody.

#8
03-11-2016
 USMLE Forums Veteran Steps History: 1+CK+CS+3 Posts: 255 Threads: 26 Thanked 85 Times in 71 Posts Reputation: 95

Although answers B through E can easily be eliminated, I am not sure that even A is correct. AS Nodo mentioned, the next step in management is often steroids.

Where is did you find this question?
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