Block 9 Epidemiology MCQ's

1. A longitudinal or prospective study is also referred to as a(n)

Ecological study

Cross-sectional study

Cohort study

Observational study

(Incidence/longitudinal/prospective study)
Starting point is definition of a group of individuals by their exposure status – exposed vs non-exposed. Group is then followed over time to see who develops the outcome of interest.
Possible to look at multiple outcomes for a given exposure.
Compare rates of disease in groups with different exposures – analytic. Rates can also be calculated for a single group and presented as a descriptive study (with no analysis of exposure).
(slide 38)

Explanation

(Incidence/longitudinal/prospective study)
Starting point is definition of a group of individuals by their exposure status – exposed vs non-exposed. Group is then followed over time to see who develops the outcome of interest.
Possible to look at multiple outcomes for a given exposure.
Compare rates of disease in groups with different exposures – analytic. Rates can also be calculated for a single group and presented as a descriptive study (with no analysis of exposure).
(slide 38)

2. When designing a study to determine whether there is a direct association between a particular exposure and an outcome, one should anticipate that potential alternative explanation(s) may exist. Which of the following is a way to deal with confounding?

Post-hoc blocking

Randomization

Screening

Validation

Random allocation or randomization is a method which allows chance and only chance to determine the assignment of subjects to the various groups
Randomization
Eliminates selection bias on the part of the participants and investigators
Tends to create groups that are comparable in terms of the distribution of known and more importantly, unknown factors that may influence the outcome (confounders)
(slide 22)

Explanation

Random allocation or randomization is a method which allows chance and only chance to determine the assignment of subjects to the various groups
Randomization
Eliminates selection bias on the part of the participants and investigators
Tends to create groups that are comparable in terms of the distribution of known and more importantly, unknown factors that may influence the outcome (confounders)
(slide 22)

3. Cedar Rivers is a community of 100,000 persons in central Iowa. During the year of the floods (2008) there were 1,000 deaths from all causes. A study of all cases of tuberculosis found the number of deaths at 300 (200 males and 100 females). During 2007, there were only 60 deaths from tuberculosis, 50 of them males. The crude mortality rate for Cedar Rivers in 2008 was:

300 per 100,000

60 per 1,000

1,000 per 100,000

100 per 11000

To calculate the crude mortality rate for Cedar Rivers in 2008, we need to determine the total number of deaths and then express it as a rate per 100,000 persons.

Total deaths in Cedar Rivers in 2008 = 1,000 Total population of Cedar Rivers = 100,000

Crude mortality rate = (Total deaths / Total population) * 100,000

Crude mortality rate = (1,000 / 100,000) * 100,000 = 1,000

Explanation

To calculate the crude mortality rate for Cedar Rivers in 2008, we need to determine the total number of deaths and then express it as a rate per 100,000 persons.

Total deaths in Cedar Rivers in 2008 = 1,000 Total population of Cedar Rivers = 100,000

Crude mortality rate = (Total deaths / Total population) * 100,000

Crude mortality rate = (1,000 / 100,000) * 100,000 = 1,000

4. A study was made of a clinician's ability to diagnose streptococcal throat infections in 149 patients coming to the emergency department in a certain hospital. The doctor's clinical impressions were compared to results of throat cultures or group A streptococcus. 37 patients had positive throat cultures and 27 of these were diagnosed by doctor as having strep throat. 112 patients had negative cultures, and the doctors diagnosed 35 of these as having strep throat. The specificity of the doctors' clinical judgment was:

27/37

77/112

27/62

10/87

104/149

The specificity of the doctors' clinical judgment refers to the ability of the doctor to correctly identify patients who do not have strep throat. In this case, out of the 112 patients with negative throat cultures, the doctor correctly diagnosed 77 of them as not having strep throat. Therefore, the specificity is calculated as 77/112.

Explanation

The specificity of the doctors' clinical judgment refers to the ability of the doctor to correctly identify patients who do not have strep throat. In this case, out of the 112 patients with negative throat cultures, the doctor correctly diagnosed 77 of them as not having strep throat. Therefore, the specificity is calculated as 77/112.

5. In a well designed clinical trial of treatment of ovarian cancer, remission rate at one year is similar for a new drug and usual care, The P-value is 0.4. This means that

Both treatments are effective

Neither treatment is effective

The statistical power of this study is 60%

The best estimate of treatment effect is 0,4

It is not possible to decide on whether one treatment is better than the other using this information alone

The P-value of 0.4 indicates that there is a 40% chance of obtaining the observed results (similar remission rates) if there was no true difference between the new drug and usual care. This means that the results are not statistically significant, and we cannot conclude that one treatment is better than the other based on this information alone. Other factors such as sample size, study design, and clinical significance should be considered before making a decision about the effectiveness of the treatments.

Explanation

The P-value of 0.4 indicates that there is a 40% chance of obtaining the observed results (similar remission rates) if there was no true difference between the new drug and usual care. This means that the results are not statistically significant, and we cannot conclude that one treatment is better than the other based on this information alone. Other factors such as sample size, study design, and clinical significance should be considered before making a decision about the effectiveness of the treatments.

6. A comparison of clinically diagnosed versus autopsy-confirmed gastric and peptc ulcers was performed on 100,000 consecutive diseased patients in several large hospitals as shown in the following table.

From these data, the sensitivity of clinical diagnosis was:

1,500/1700

200/96,000

95,800/96,000

1,500/4,000

2,500/4,000

The sensitivity of clinical diagnosis is calculated by dividing the number of true positive cases (clinically diagnosed cases that were confirmed by autopsy) by the total number of actual positive cases (clinically diagnosed cases). In this case, the number of true positive cases is given as 1,500 and the total number of clinically diagnosed cases is given as 4,000. Therefore, the sensitivity of clinical diagnosis is 1,500/4,000.

Explanation

The sensitivity of clinical diagnosis is calculated by dividing the number of true positive cases (clinically diagnosed cases that were confirmed by autopsy) by the total number of actual positive cases (clinically diagnosed cases). In this case, the number of true positive cases is given as 1,500 and the total number of clinically diagnosed cases is given as 4,000. Therefore, the sensitivity of clinical diagnosis is 1,500/4,000.

7. The mode of transport of an infectous agent through the environment to a susceptible host is called a:

Carrier

Reservoir

Vector

Vehicle

Vehicle: Mode of transport of an infectious agent through the environment to a susceptible host.

Vector: A living carrier – usually an arthropod – that serves as a mode of transport for an infectious agent from an infected host to a susceptible host.

Reservoir: The primary habitat in which an infectious agent survives and reproduces
(slide 7)

Explanation

Vehicle: Mode of transport of an infectious agent through the environment to a susceptible host.

Vector: A living carrier – usually an arthropod – that serves as a mode of transport for an infectious agent from an infected host to a susceptible host.

Reservoir: The primary habitat in which an infectious agent survives and reproduces
(slide 7)

8.

After attending a retirement party for the Behavioral Sciences Department Chairperson, many of the faculty developed gastroenteritis. All attendees were interviewed by the local public health nurse who had completed a Principles of Epidemiology self-study course. The table shows the results of the interviews. Calculate the appropriate measure of association for each of the home-made food items shown in the table above. For which food is the measure of association the largest?

Macaroni Salad

Potato Salad

Three-bean Salad

Ice Cream

The measure of association for each food item is calculated by dividing the number of people who ate the food and developed gastroenteritis by the number of people who ate the food but did not develop gastroenteritis. The measure of association is the highest for Ice Cream, indicating that there is a stronger association between consuming Ice Cream and developing gastroenteritis compared to the other food items.

Explanation

The measure of association for each food item is calculated by dividing the number of people who ate the food and developed gastroenteritis by the number of people who ate the food but did not develop gastroenteritis. The measure of association is the highest for Ice Cream, indicating that there is a stronger association between consuming Ice Cream and developing gastroenteritis compared to the other food items.

9. A laboratory test has been developed to assess the risk of prostate cancer among men. Studies with human subjects involving repeated measures indicate that the test yields the same results for every individual again and again, However, other studies indicate that there is a very low correlation between this new test for prostate cancer and already- existing tests with proven track records. The areas of relative strength and relative weakness of this new test respectively are:

Sensitivity and Specificity

Specificity and sensitivity

Reliability and validity

Validity and reliability

The given question is asking about the areas of relative strength and relative weakness of the new test for prostate cancer. The statement mentions that the test yields the same results for every individual repeatedly, indicating high reliability. However, it also mentions that there is a very low correlation between this new test and already-existing tests with proven track records, indicating low validity. Therefore, the relative strength of the new test is reliability, while its relative weakness is validity.

Explanation

The given question is asking about the areas of relative strength and relative weakness of the new test for prostate cancer. The statement mentions that the test yields the same results for every individual repeatedly, indicating high reliability. However, it also mentions that there is a very low correlation between this new test and already-existing tests with proven track records, indicating low validity. Therefore, the relative strength of the new test is reliability, while its relative weakness is validity.

10. All of the following are true of odds ratio except:

It is an estimate of relative risk

It is the only measure of risk that can be obtained directly form a case-control study

It tends to be biased towards 1 (neither risk or protection) at high rates of disease

It is the ratio of incidence in exposed divided by incidence in nonexposed

It can be calculated without data on rates (as in a case-control study)

it is the ODDS of diseas not the INCIDENCE

IF
risk in exposed = p1, and
risk in unexposed = p0
then
risk ratio = p1/p0
IF
odds of disease in exposed = odds1, and odds of disease in unexposed = odds0
Then
odds ratio = odds1/odds0

Explanation

it is the ODDS of diseas not the INCIDENCE

IF
risk in exposed = p1, and
risk in unexposed = p0
then
risk ratio = p1/p0
IF
odds of disease in exposed = odds1, and odds of disease in unexposed = odds0
Then
odds ratio = odds1/odds0

11. A study was made of a clinician's ability to diagnose streptococcal throat infections in 149 patients coming to the emergency department in a certain hospital. The doctor's clinical impressions were compared to results of throat cultures or group A streptococcus. 37 patients had positive throat cultures and 27 of these were diagnosed by doctor as having strep throat. 112 patients had negative cultures, and the doctors diagnosed 35 of these as having strep throat. The predictive value of the doctors' clinical judgment for streptococcal sore throat was:

27/37

77/112

27/62

10/87

104/149

The predictive value of the doctors' clinical judgment for streptococcal sore throat can be calculated by dividing the number of true positive diagnoses (27) by the sum of true positive diagnoses and false positive diagnoses (27+35). This gives us a fraction of 27/62, indicating that the doctors' clinical judgment correctly diagnosed streptococcal sore throat in 27 out of 62 cases.

Explanation

The predictive value of the doctors' clinical judgment for streptococcal sore throat can be calculated by dividing the number of true positive diagnoses (27) by the sum of true positive diagnoses and false positive diagnoses (27+35). This gives us a fraction of 27/62, indicating that the doctors' clinical judgment correctly diagnosed streptococcal sore throat in 27 out of 62 cases.

12. A screening test for brain cancer was administered to 400 women with biopsy-proven brain cancer and to 400 women without brain cancer. The test results were positive for 100 of the proven cases and 50 of the normal women. The predictive value of a positive test is

88%

33%

25%

67%

true positives/ all positive or 100/100+50 = 2/3 or 67%

Explanation

true positives/ all positive or 100/100+50 = 2/3 or 67%

13. The following drug trial is synonymous with the term "clinical trial"?

Clinical pharmacology and toxicity

Full scale Evaluation of Treatment

Initial Clinical Investigation for Treatment Effect

Post marketing Survelillance

2 types of intervention study
Field trials: trials to prevent disease. Involves evaluation of whether an agent or procedure reduces the risk of developing disease among those free from that condition at enrolment. Logistically more difficult to carry out as compared to a clinical trial.
Clinical trials: trials to evaluate one or more new treatments for a disease or condition. Since clinical trials involve diseased persons they are often carried out in hospitals or other clinical settings where those enrolled are treated and followed up for their condition.
(slide 4)

Explanation

2 types of intervention study
Field trials: trials to prevent disease. Involves evaluation of whether an agent or procedure reduces the risk of developing disease among those free from that condition at enrolment. Logistically more difficult to carry out as compared to a clinical trial.
Clinical trials: trials to evaluate one or more new treatments for a disease or condition. Since clinical trials involve diseased persons they are often carried out in hospitals or other clinical settings where those enrolled are treated and followed up for their condition.
(slide 4)