# Statistics Quiz

**New Stat Quiz**

- A screening test is used in the same way in two similar populations, but the proportion of false-positive results among those who test positive in population A is higher than that among those who test positive in population B. What is the most likely explanation for this finding?

The specificity of the test is higher in population A | ||

The specificity of the test is lower in population A | ||

The prevalence of the disease is higher in population A | ||

The prevalence of the disease is lower in population A |

2.

SAMPLE 2 BY 2 TABLE | |||

Outcome | Total | ||

Factor | + | − | |

+ | A | B | A + B |

− | C | D | C + D |

Total | A + C | B + D | A + B + C + D |

Assuming that the sample table is for a cohort study, define the risk difference or attributable risk.

(A/A+C) / (B/B+D) | ||

(A/A+B) / (C/C+D) | ||

(A/A+C) − (B/B+D) | ||

(A/A+B) – (C/C+D) | ||

None of the above |

3. If it is accepted that an observed association is a causal one, an estimate of the impact that a successful preventive program might have can be derived from:

higher life expectancy |
||

attributable risk | ||

prevalence rates | ||

relative risk | ||

all of the above |

** **4. A new screening test for Lyme disease is developed for use in the general population. The sensitivity and specificity of the new test are 60% and 70%, respectively. Three hundred people are screened at a clinic during the first year the new test is implemented. Assume the true prevalence of Lyme disease among clinic attendees is 10%.

The predictive value of a negative test is:

33.0% | ||

18.2% | ||

94.0% | ||

22.2% | ||

6.0% |

- A screening examination was performed on 250 persons for Factor X, which is found in disease Y. A definitive diagnosis for disease Y among the 250 persons had been obtained previously. The results are charted below:

RESULTS OF DIAGNOSIS | ||

TEST RESULTS |
Disease Present |
Disease Absent |

Positive for Factor X | 40 | 60 |

Negative for Factor X | 10 | 140 |

The specificity of this test is expressed as:

56% |
||

30% | ||

7% | ||

80% | ||

70% |

- A new blood test has been developed to screen for disease Z. Researchers establish 50 units as a cut point above which a test is considered positive and thereby indicative of disease. The test manufacturers determine that the test’s sensitivity is unacceptably low. However, the manufacturers are not concerned with the specificity and do not want the cost of the test to rise. How can they improve the sensitivity of the test?

Lower the cut point below 50 units. |
||

They cannot improve this test and should begin work developing a new test. | ||

Rise the cut point above 50 units. | ||

Test each person’s blood twice. |

- Sensitivity and specificity of a screening test refer to its:

Reliability |
||

Validity | ||

Yield | ||

Repeatability | ||

None of the above |

8. The figure on page 475 represents different combinations and qualities of validity and reliability (high vs. low).

Which set of symbols represents low reliability?

A | ||

B | ||

C | ||

Both A and C | ||

None of the above |

9. The figure on page 475 represents different combinations and qualities of validity and reliability (high vs. low).

Which set of symbols represents high reliability?

B | ||

None of the above | ||

Both A and C | ||

C | ||

A |

- Specificity refers to the ability of a screening test to identify only non-diseased individuals who actually do not have the disease.

True

False

- Phase III clinical trials for a cancer drug involve:

comparing survival rates for the new drug versus extant therapies |
||

None of the answers listed here | ||

initial testing in humans | ||

testing with different tumor types |

** ****Solution**** **** **

Q1.

A screening test is used in the same way in two similar populations, but the proportion of false-positive (FP) results among those who test positive (FP + TP) in population A is higher than that among those who test positive in population B.

When the prevalence of a disease is higher in one population than another, the total with the disease is higher (TP + FN is higher), and the total without the disease is lower (FP+TN is lower). Given the same sensitivity and specificity, the proportion of FP to FP+TP will decrease (fewer false positives).

Since the proportion of FP to FP + TP is higher in population A than in population B, the prevalence of the disease is lower in population A.

Hence, the answer is – the prevalence of the disease is lower in population A

Q2.

SAMPLE 2 BY 2 TABLE | |||

Outcome | Total | ||

Factor | + | − | |

+ | A | B | A + B |

− | C | D | C + D |

Total | A + C | B + D | A + B + C + D |

Assuming that the sample table is for a cohort study, the risk difference or attributable risk is defined by. –

Hence, the answer is (A/A+B) – (C/C+D)

Q3.

If it is accepted that an observed association is a causal one, an estimate of the impact that a successful preventive program might have can be derived from attributable risk.

Hence, the answer is – attributable risk

Q4.

A new screening test for Lyme disease is developed for use in the general population. The sensitivity and specificity of the new test are 60% and 70%, respectively. Assume the true prevalence of Lyme disease among clinic attendees is 10%.

Three hundred people are screened at a clinic during the first year the new test is implemented. Since the true prevalence of Lyme disease is 10%, 30 people actually have Lyme disease and 270 people screened do not have Lyme disease.

Since the sensitivity of the new test is 60%, 60% of persons actually with Lyme disease will be screened by the test as having the disease. So, 18 (60% * 30) people having Lyme disease will be screened by the test as having the disease and 30 – 18 = 12 people having Lyme disease will be screened negative by the test.

Since the specificity of the new test is 70%, 70% of persons actually not having Lyme disease will be screened negative by the test. So, 189 (70% * 270) people not having Lyme disease will be screened by the test as having the disease.

The predictive value of negative test is

= = = 94%

Hence, the answer is – 94.0%

Q5.

A screening examination was performed on 250 persons for Factor X, which is found in disease Y. A definitive diagnosis for disease Y among the 250 persons had been obtained previously.

RESULTS OF DIAGNOSIS | ||

TEST RESULTS |
Disease Present |
Disease Absent |

Positive for Factor X | 40 | 60 |

Negative for Factor X | 10 | 140 |

Specificity is the fraction of those without the disease who will have a negative test result.

So, specificity =

= = 70%

Hence, the answer is – 70%

Q6.

A new blood test has been developed to screen for disease Z. Researchers establish 50 units as a cut point above which a test is considered positive and thereby indicative of disease. The test manufacturers determine that the test’s sensitivity is unacceptably low. However, the manufacturers are not concerned with the specificity and do not want the cost of the test to rise.

The sensitivity of the test is low implies that the proportion of people actually having the disease Z screened positive by the test is low. To increase the sensitivity of the test, we need to increase the number of positive test results. Since the test manufacturers are not concerned with the specificity and do not want the cost of the test to rise, they can lower the cut point below 50 units to increase the sensitivity of the test.

Hence, the answer is – lower the cut point below 50 units

Q7.

The sensitivity and specificity of a screening test refer to its validity.

Hence, the answer is – validity.

Q8.

Reliability is the degree to which a test produces stable and consistent results.

The symbol B represents low reliability.

Hence, the answer is – B

Q9.

Reliability is the degree to which a test produces stable and consistent results.

The symbols A and C represent high reliability.

Hence, the answer is – Both A and C

Q10.

Specificity measures the proportion of negatives that are correctly identified as such (e.g. the percentage of healthy people who are correctly identified as not having the condition). Thus, specificity refers to the ability of a screening test to identify only non-diseased individuals who actually do not have the disease. So, the given statement is True.

Hence, the answer is – True

Q11.

Phase III clinical trials compare the safety and effectiveness of the new treatment against the current standard treatment.

So, Phase III clinical trials for a cancer drug involve comparing survival rates for the new drug versus extant therapies.

Hence, the answer is – comparing survival rates for the new drug versus extant therapies