Many parts of the developing world have limited access to clean and safe drinkin

Question

Many parts of the developing world have limited access to clean and safe drinking water. There is a gold standard test that can be performed in a laboratory to tell for certain whether there are harmful pathogens in drinking water. However, this test requires trained researchers, expensive equipment, and takes several days. Public health researchers have developed a new screening test that anyone can perform, using inexpensive filter paper that, when dipped in a water sample for 5 seconds, will indicate by color the presence of harmful pathogens. If, after proper dipping the filter paper remains white, the water is safe to drink. If the filter paper turns any shade of orange or red, pathogens are present and water should be boiled prior to consumption.

Public health researchers get permission to evaluate this new screening test in a community in Bangladesh, where water contamination is a frequent problem. Researchers collect water samples from the main community well three times a day for two weeks, a total of 42 samples. A portion of each of these samples is sent to the lab to undergo the gold standard test, while the remainder of each sample is tested using the new filter paper method. The gold standard laboratory test reveals that 21 of the 42 water samples are truly contaminated. The filter paper screening method results in 24 positive test results. Of these filter paper results, 19 are confirmed to be true positives.

Question 25 options:

Using this information, draw and complete a 2 x 2 table.

Truth

   Filter Paper Test        Positive       Negative     Totals

              Positive               

                                   

           Negative               

                                     

              Total                    

Using the above information, calculate the sensitivity and specificity for the new filter paper test. Round to the nearest whole number. Express all of your answers as a percent (%)

Sensitivity = TP / (TP + FN):

Specificity = TN / (FP = TN):

Question 26 options:

Using the above information, calculate the positive predictive value (PPV). Round to the nearest whole number. Express all of your answers as a percent (%)

PPV = TP / (TP + FP):

Based on your calculations for sensitivity above, select the correct interpretations for this calculation from the options below. There may be more than one correct interpretation listed.

Question 28 options:

A)

Out of 42 samples, 91% of them were contaminated.

B)

The filter paper test will be correct 91% of the time.

C)

91% of the samples that were truly contaminated were correctly identified by the filter paper screening test.

D)

Of the water samples tested, 91% of them will test positive for contamination.

E)

The proportion of water samples with pathogens present that was correctly identified as such by the filter paper screening method was 91%.

Explanation / Answer

Confusion matrix:

Excel formula:

Based on your calculations for sensitivity above, select the correct interpretations for this calculation from the options below. There may be more than one correct interpretation listed.

Question 28 options:

correct ans is: E

Actual Positive Negative Predicted Positive 19 5 24 Negative 2 16 18 21 21 42 Actual Positive Negative Predicted Positive TP FP Negative FN TN

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