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#1




Biostatistics #18  Screening tests
A screening test for breast cancer was administered to 400 women with biopsyproven breast cancer and to 400 women without breast cancer. The test results were positive for 100 of the proven cases and 50 of the normal women.
1. What is the probability that the patient has the disease, given a positive test? a. 88% b. 67% c. 25% d. 33% e. 12% 2. What is the probability that the test is positive, given that the patient has the disease? a. 88% b. 67% c. 25% d. 33% e. 12% 3. What is the probability that the test is positive, given that the patient don't have the disease? a. 88% b. 67% c. 25% d. 33% e. 12% 
#2




1. D
2. C 3. C I'm really having a hard time with these... Thanks for all the questions!
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bebix (06102011) 
#3




i dont know!!

#4




@pass7...just relax, take a deep breath and try again...

#7




Quote:
2.c 3.a 
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bebix (06102011) 
#9




correct answer
Correct answers:
1B 2C 3E Gold Standard (GS) = biopsy (Disease = D) First, let's create a 2x2 table: ..........Disease(+)....Disease() Test(+).....100...........50.......TOTAL = 150 Test().....300..........350.......TOTAL = 650  ...............400..........400.......TOTAL = 800 1.What is the probability that the patient has the disease, given a positive test? = PPV => P(D+) P(D+)  numerator Disease AND test (+)  denominator ALL test (+) = 100/150 = 2/3 or 67% 2.What is the probability that the test is positive, given that the patient has the disease? = Sensitivity => P(+D) P(+D)  numerator test (+) AND Disease  denominator ALL Disease(+) = 100/400 = 1/4 or 25% 3.What is the probability that the test is positive, given that the patient don't have the disease? = 1Specificity => P(+no D) P(+no D)  numerator test (+) AND NO Disease  denominator ALL Disease() = 50/400 = 1/8 or 12.5% 
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aknz (10082011), Dr_Laura (06102011), drortho (11232011), excellence (05182014), nevillenic (06102011), pass7 (06102011), steps_8186 (04112012) 
#10




Thanks so much! This has helped me a lot
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bebix (06102011) 
#11




Quote:

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bebix (06102011) 
#12




#3 question is 88%.
Trues are always on top. 350/400=88% 
#13




no evavar...positives are on top......and true positives are on top and true negatives are on bottom..

#14




Specificity = Probability of a NEGATIVE test given the person DON'T have the Disease
P(no D) = 350/400 = 88% 1  Specificity = Probability of a POSITIVE test given the person DON'T have the Disease P(+no D) = 50/400 = 1/8 or 12.5% I never use true positive, true negative....this is just a conditional probability (something GIVEN something else)...so, the "something else" MUST be in the DENOMINATOR. Hope this help 
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pass7 (06102011) 
#15




Bebix, you are right about taking a deep breath.
On these questions, make sure to read the question carefully and understand what is being asked. In these questions, we are given the exact parameters we need to answer the questions without having any knowledge of sensitivity, specificity, PPV, NPV, etc. 1. Of the people who tested positive (150 tested positive), how many had the disease (100), therefore 100/150 = 2/3 = 67% 2. Given the patient has the disease (400 people fall into this category, how many tested positive (100 with the disease tested positive). Therefore 100/400 = 25% 3. Given the patient doesnt have the disease (400 don't), how many tested positive (50 tested positive when they didnt have the disease). Therefore 50/400 = 12% They are going to try to trick us with the wording on these types of questions so take the extra 30 seconds to reword/rephrase the question so that it makes more sense to you. 
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excellence (05182014) 
#16




Quote:
Biostatistics #19  Screening tests (part b) 
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rulz (10062011) 
#17




Quote:
i think last one is tricky ,, he is asking about false positve rate FP rate= 1  specificity so 1st we find out specificity , that's trues are on top divided by everything in that group so specificity= 350/400 = 88% now FPrate is =188=12% so i think answer is clear 
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