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#1
06-10-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,259 Times in 881 Posts Reputation: 3269
Biostatistics #18 - Screening tests

A screening test for breast cancer was administered to 400 women with biopsy-proven 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%
 The above post was thanked by: aknz (10-08-2011), Al-Saoudi (06-10-2011), excellence (05-18-2014), pass7 (06-10-2011), Rsadhu (01-13-2015), rulz (10-06-2011)

#2
06-10-2011
 USMLE Forums Guru Steps History: 1+CK+CS Posts: 499 Threads: 85 Thanked 529 Times in 222 Posts Reputation: 539

1. D
2. C
3. C

I'm really having a hard time with these... Thanks for all the questions!
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 The above post was thanked by: bebix (06-10-2011)
#3
06-10-2011
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i dont know!!
#4
06-10-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,259 Times in 881 Posts Reputation: 3269

Quote:
 Originally Posted by pass7 i dont know!!
@pass7...just relax, take a deep breath and try again...
#5
06-10-2011
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answer may
b.
c.
d.
 The above post was thanked by: bebix (06-10-2011)
#6
06-10-2011
 USMLE Forums Veteran Steps History: 1 + CK Posts: 229 Threads: 6 Thanked 68 Times in 61 Posts Reputation: 78

1. A
2. c
3. e
 The above post was thanked by: bebix (06-10-2011)
#7
06-10-2011
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Quote:
 Originally Posted by bebix a screening test for breast cancer was administered to 400 women with biopsy-proven 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%
1.b
2.c
3.a
 The above post was thanked by: bebix (06-10-2011)
#8
06-10-2011
 USMLE Forums Newbie Steps History: Not yet Posts: 2 Threads: 0 Thanked 3 Times in 2 Posts Reputation: 13
nice

1= positive pred value= 66.67%
2=sensitivity= 25%
3= false positive rate= 1-specificity= 1-.12=88%
 The above post was thanked by: bebix (06-10-2011)
#9
06-10-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,259 Times in 881 Posts Reputation: 3269
correct answer

Correct answers:
1-B
2-C
3-E

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? = 1-Specificity => P(+|no D)

P(+|no D)
- numerator test (+) AND NO Disease
- denominator ALL Disease(-)

= 50/400 = 1/8 or 12.5%
 The above post was thanked by: aknz (10-08-2011), Dr_Laura (06-10-2011), drortho (11-24-2011), excellence (05-18-2014), nevillenic (06-10-2011), pass7 (06-10-2011), steps_8186 (04-11-2012)
#10
06-10-2011
 USMLE Forums Guru Steps History: 1+CK+CS Posts: 499 Threads: 85 Thanked 529 Times in 222 Posts Reputation: 539

Thanks so much! This has helped me a lot
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 The above post was thanked by: bebix (06-10-2011)
#11
06-10-2011
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Quote:
 Originally Posted by bebix Correct answers: 1-B 2-C 3-E 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? = 1-Specificity => P(+|no D) P(+|no D) - numerator test (+) AND NO Disease - denominator ALL Disease(-) = 50/400 = 1/8 or 12.5%
ohh..thnks....i was just near to answer..only 3rd was wrong....just slip in question to understand and made it wrong..hope this type of sily mistake will not occur in real exam.....thnks bebix for such good question..
 The above post was thanked by: bebix (06-10-2011)
#12
06-10-2011
 USMLE Forums Guru Steps History: 1 + CS Posts: 438 Threads: 111 Thanked 231 Times in 132 Posts Reputation: 241

#3 question is 88%.
Trues are always on top.
350/400=88%
#13
06-10-2011
 USMLE Forums Scout Steps History: Not yet Posts: 34 Threads: 9 Thanked 14 Times in 11 Posts Reputation: 24

Quote:
 Originally Posted by evavar #3 question is 88%. Trues are always on top. 350/400=88%
no evavar...positives are on top......and true positives are on top and true negatives are on bottom..
#14
06-10-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,259 Times in 881 Posts Reputation: 3269

Quote:
 Originally Posted by evavar #3 question is 88%. Trues are always on top. 350/400=88%
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
 The above post was thanked by: pass7 (06-10-2011)
#15
06-10-2011
 USMLE Forums Guru Steps History: Not yet Posts: 298 Threads: 45 Thanked 713 Times in 210 Posts Reputation: 723

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.
 The above post was thanked by: excellence (05-18-2014)

#16
06-10-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,259 Times in 881 Posts Reputation: 3269

Quote:
 Originally Posted by apx85 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.
yup, math never change...I think is better to understand the concept, rather than memorize weird formulas...here is a new one

Biostatistics #19 - Screening tests (part b)
 The above post was thanked by: rulz (10-06-2011)
#17
01-12-2015
 USMLE Forums Scout Steps History: CK+CS Posts: 87 Threads: 22 Thanked 12 Times in 12 Posts Reputation: 22

Quote:
 Originally Posted by bebix A screening test for breast cancer was administered to 400 women with biopsy-proven 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%

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 =1-88=12%

so i think answer is clear

 Tags Biostatistics-Epidemiology, Step-1-Questions

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