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  #1  
Old 02-06-2012
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Default what effect on the screening

Q. The National Council for the Prevention of Violence (NCPV) has long recommended that primary care physicians screen for the presence of guns in the homes of their patients. The recommended screening question is, Do you have any type of gun or firearm in the home where you live? Recently, the Council has recommended a second question be added to this original question. The second recommended question is, Do you keep your firearm locked so that a key is required to access it? If patient's response affirmatively to the first question and negatively to the second, the Council recommends that the physician spends a couple of minutes reviewing some of the key aspects of firearm safety. Materials for this discussion are provided free of charge on the Council's website. This change in recommendation by the Council is most likely to have what effect on the screening for potential gun violence?
A. Cannot be determined with any certainty from the information given
B. Decreased positive predictive value
C. Decreased sensitivity
D. Decreased specificity
E. Increased efficiency of the screening procedure
F. Increased negative predictive value
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Old 02-06-2012
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Ans ........ B ..........

well how i see this is that this will tell about the safety of the gun ,
so will increase the number of people who have kept the gun without a lock .

so if you make a 4 * 4 block with one side having the gun or not and other side with or without the lock you will find a increase in the place of FP (b) ie people with the gun and without the lock and decrease in TP ie people with the gun and with the lock

so the PPV will decrease .......

No idea actually how to do this .........
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Old 02-06-2012
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I think its B too.
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Old 02-08-2012
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The answer is C.
Moving from a one to a two question sequence will decrease the sensitivity, increase the specificity, increase the positive predictive value, and decrease the negative predictive value of the screening test. The two-question sequence makes it harder to classify someone as at risk. Certain people who would have been called at risk by a yes to question 1 will now be excluded if they say yes to question 2. Excluding more people reduces the sensitivity but increases specificity. However, people who have a gun and keep it unlocked are more likely to use the gun in violence than if they keep the gun locked so, positive predictive value increases, while negative predictive value decreases. The impact on efficacy (accuracy) is difficult to determine without more information, so this cannot be the best answer.
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Old 02-08-2012
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hint,

generally when it comes to the "cutoff" issue,the directions of changes are similar in this way:

Spec.~PPV
Sens.~NPV

in this question,they have simply raised the cutoff.
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