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  #1  
Old 01-21-2014
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Stats Two False Positive/Negative Questions

1) A city with 1000 citizens has a 10% prevalence of disease x ..
Screening shows sensitivity 80% and specificity 70%
How many people without disease X will be falsely diagnosed positive by this screening test..
1) 20
2) 30
3) 270
4) 300


2) A city with 1000 citizens has a 10% prevalence of disease x ..
Screening shows sensitivity 80% and specificity 70%
How many people with disease X will be missed by this screening??
1) 20
2) 30
3) 270
4) 300

Last edited by medicalbiology; 01-21-2014 at 09:49 PM.
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  #2  
Old 01-21-2014
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Default

1) c

2) a
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Old 01-22-2014
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Default

1. 270

2. 20
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Old 01-22-2014
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Default

................Disease
...........+............ -
Test + 80 ............ 270

....... - 20 .............630

..........100 .......... 900
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Old 02-03-2014
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Default Explanations, please

Quote:
Originally Posted by medicalbiology View Post
1) A city with 1000 citizens has a 10% prevalence of disease x ..
Screening shows sensitivity 80% and specificity 70%
How many people without disease X will be falsely diagnosed positive by this screening test..
1) 20
2) 30
3) 270
4) 300


2) A city with 1000 citizens has a 10% prevalence of disease x ..
Screening shows sensitivity 80% and specificity 70%
How many people with disease X will be missed by this screening??
1) 20
2) 30
3) 270
4) 300
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  #6  
Old 02-03-2014
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Default Explanation

Always factor in prevalence. In this case, prevalence is 10% of 1000 people i.e. Diseased = 100 Non Diseased = 900 (1000-100)

TP = Sensitivity * Diseased people
= 80/100 * 100
= 80

TN = Specificity * Non Diseased
= 70/100 * 900
= 630

Thus, FP = Non Diseased - TN ( Those who were incorrectly identified as having the disease)
= 900 - 630
= 270 (answer 1)

Plug these values into a 4*4 table,

TP ------ FP
FN-------TN

Total has to equal 1000 (total population measured). Hence, those incorrectly diagnosed as not having the disease/ those this test misses = 20 (answer 2)
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Old 02-04-2014
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Default Thx a million!!!

thx and i really appreciated your explanations.
.
Quote:
Originally Posted by Doc889 View Post
Always factor in prevalence. In this case, prevalence is 10% of 1000 people i.e. Diseased = 100 Non Diseased = 900 (1000-100)

TP = Sensitivity * Diseased people
= 80/100 * 100
= 80

TN = Specificity * Non Diseased
= 70/100 * 900
= 630

Thus, FP = Non Diseased - TN ( Those who were incorrectly identified as having the disease)
= 900 - 630
= 270 (answer 1)

Plug these values into a 4*4 table,

TP ------ FP
FN-------TN

Total has to equal 1000 (total population measured). Hence, those incorrectly diagnosed as not having the disease/ those this test misses = 20 (answer 2)
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