Question at position 3 Based on the data table and the answers given so far, is bleaching a water supply with a positive result a good use of resources? You must pick two: one statement if there is another test and one statement if not.yes, if there is another assay test availableno, if there is another assay availablemaybe, if there is another assay availableyes, if there is no other assay availableno, if there is no other assay available多项选择题
A
yes, if there is another assay test available
B
no, if there is another assay available
C
maybe, if there is another assay available
D
yes, if there is no other assay available
E
no, if there is no other assay available

Question

Question at position 3 Based on the data table and the answers given so far, is bleaching a water supply with a positive result a good use of resources? You must pick two: one statement if there is another test and one statement if not.yes, if there is another assay test availableno, if there is another assay availablemaybe, if there is another assay availableyes, if there is no other assay availableno, if there is no other assay available多项选择题

A

yes, if there is another assay test available

B

no, if there is another assay available

C

maybe, if there is another assay available

D

yes, if there is no other assay available

E

no, if there is no other assay available

BlackTom AI · Accepted Answer

1. Relevant knowledge points and concepts
- Diagnostic test characteristics: sensitivity, specificity, prevalence, and positive predictive value (PPV). PPV determines the probability a positive test truly indicates contamination.
- Risk–benefit and resource-allocation principles: costs of bleaching (money, labor, disruption) vs public-health benefit of preventing exposure.
- Value of confirmatory testing: a second assay increases confidence before a costly intervention.

2. Step-by-step reasoning
- A single positive assay can be a true positive or a false positive. The likelihood it is true depends on PPV.
- PPV = (sensitivity × prevalence) / [(sensitivity × prevalence) + (1 − specificity) × (1 − prevalence)].
- If prevalence is low and specificity <100%, PPV can be low → many positives will be false. In that case, performing another independent assay before using resources to bleach is sensible (hence “maybe” if another assay is available).
- If no alternative assay exists, you must weigh uncertainty against the health risk. For serious contamination with high potential harm, the precautionary principle favors treating (bleaching) despite uncertainty — so “yes” if there is no other assay available.

3. Example calculation (intermediate steps)
- Assume prevalence 1%, sensitivity 95%, specificity 95%:
  - Numerator = 0.95 × 0.01 = 0.0095
  - False-positive term = (1 − 0.95) × 0.99 = 0.05 × 0.99 = 0.0495
  - PPV = 0.0095 / (0.0095 + 0.0495) = 0.0095 / 0.059 = 0.161 ≈ 16%
- With PPV ≈16%, most positives are false → confirmatory testing advisable if available.

4. Why the correct answers are correct
- “Maybe, if there is another assay available”: confirms the need to consider confirmatory testing and context; bleaching is conditional on confirmatory results and risk tolerance.
- “Yes, if there is no other assay available”: with no way to confirm and potential severe harm, precaution supports bleaching.

5. Why other options are incorrect
- “Yes, if there is another assay available”: wrong because you shouldn’t automatically expend resources before confirming.
- “No, if there is another assay available”: wrong because a positive test should prompt at least confirmation and possibly action depending on results.
- “No, if there is no other assay available”: wrong because it ignores precaution when confirmation is impossible and risk to public health may be high.

6. Conclusion
The recommended choices—“maybe, if there is another assay available” and “yes, if there is no other assay available”—match the provided answers.

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