Background information:

A panel of experts reviewed the medical records of 525 patients discharged from the hospital with diagnosis codes indicative of a stroke (ICD 430-438). The panel classified strokes as either ischemic or not ischemic. Assume the diagnos is reached by the panel is the most accurate classification possible. Of the 525 cases, 325 had a discharge diagnosis code for ischemic stroke (ICD code 434). Of these 325 patients, 85 were determined by the panel not to be ischemic strokes. All but 20 o f the patients with discharge diagnosis codes other than 434 were determined by the panel to have non-ischemic strokes.

Given the background information, compute the sensitivity, specificity, and positive predictive value of a hospital discharge code for ischemic stroke (ICD code 434) in classifying a patient as truly having an ischemic stroke.

1. sensitivity of a 434 code: (2 pts)



2. specificity of a 434 code: (2 pts)



3. positive predictive value of a 434 code: (2 pts)



4. Constructing a receiver/response operating characteristic (ROC) curve may be useful in understanding the implications of using different case definitions. Briefly explain what a ROC curve is and what information it provides. (2 pts)



5. If you were to use a 434 discharge code to identify a group of cases with ischemic stroke and the sensivity was 99% but the specificity was 40%, which of the following would best describe your resulting case group. (choose one best answer). (2 pts)

1. The case group would be highly homogenous with respect to pathophysiology of stroke.
2. The case group would be highly heterogeneous with respect to pathophysiology of stroke.
3. The case group would have many false negative ischemic strokes.
4. The case group would represent the source population of cases.

6. What two factors influence the positive predictive value of a screening test in most situations? (2 pts)

9. Suppose that a study was conducted to compare the rates of automobile collisions in two cities. The researchers were impressed with studies that suggest that the use of cell phones and pagers contribute to auto collisions. They wanted to adjust (standardize) the rates of auto collisions in the two cities for cell phone and pager use. Data on cell phone use and auto collisions in the two cities were collected and are presented in the table below.

                        * per 1000 persons

1. Calculate the crude total and cell phone/pager use specific rates for Corona del Mar and Boulder. How do these two cities compare in crude prevalence of auto accidents. (2 pts)



2. Using the combined number of persons in both areas as a standard, calculate a standardized rate (standardized for cell phone/pager use) for each of the states. Use the direct standardization method. Briefly describe how these standardized rates compare with each other and with the crude rates. Briefly describe any meaningful differences. (4 pts)



3. In general, which of the following best describes a major weakness of both crude and adjusted rates? (2 pts)

10. In a community intervention study, like the Minnesota Heart Health Program, the effectiveness of an educational intervention program was evaluated. Which of the following best describes the unit of assignment, the unit of observation, and the unit of analysis in these types of studies (in this order)? (2 pts)

1. group, person, group
2. person, group, group
3. group, group, group
4. none of the above

11. Indicate next to each statement below whether you consider it to be TRUE, FALSE, or if you are NOT SURE. A correct answer receives 2 points, an incorrect one zero.

1. An advantage of cohort designs compared to the pure case control designs is that cohort studies can directly estimate risks.
2. The temporal sequence of exposure and disease can be directly addressed in a cohort design as well as in a case control study.
3. A disadvantage of the cohort design compared to a case control study design is that in a cohort study one cannot address multiple outcomes.
4. As described in class, a randomized clinic trial is an example of a prospective dynamic cohort study.
5. A disadvantage of the cohort design compared to a case control study is that in a cohort study one needs to follow a large number of participants if the disease is rare.
6. Ecological studies cannot directly assess causal inference because they measure disease and exposure in a person at the same point in time.
7. Correlation studies can be quick, inexpensive, and allow for multinational comparisons.
8. A case report is a type of descriptive study that is commonly conducted, partially because an appropriate control group is easily defined.
9. Cross-sectional studies are limited by their lack of generalizability, but are powerful in that they directly measure risk.
10. The study of person, place, and time helps to understand the natural history of a disease.
11. A risk difference is determined by the absolute difference in two incidence rates, whereas the relative difference is considered an attributable risk.
12. A correlation coefficient measures the degree of linear or monotonic relationship between two variables and is therefore suitable for determining the epidemiologic strength of association between them.
13. As an estimate of a relative risk, an odds ratio is a measure of association that can be used to determine the magnitude of an association between exposure and an outcome.
14. An attributable risk proportion is a measure of the impact assessing how much risk results from exposure levels. Attributable risks that adjust for the prevalence of the causal factor in a population is called a population attributable risk.
15. Case control studies have several crucial advantages that relate to their efficiency for studying rare conditions and those with prolonged induction and their efficiency in examining many exposure and outcomes.
16. Incidence density is a proportion where the units of time are specified.
17. The decision to use an incidence density measure or a cumulative incidence as a measure of the strength of association may depend on the objectives of the study. Cumulative incidence is preferred if estimating individual risk is the main objective.
18. A standardized mortality ratio (SMR) can be determined using indirect adjustment. Because rates from a standard population are used, SMR’s from two study populations can be compared as long as the rates in the standard population are stable.
19. Comparability between cases and controls is a important step in constructing a case-control study. It should be possible to detect exposure in controls to the same extent as in cases. It is also critical that controls have similar motivation and availability as cases. These two conditions are best met when controls are selected from the general population.

12. Attributable measures are used by researchers to assess the public health impact of a detrimental exposure, assuming causality. Given data from a cohort study on the incidence of stroke (see below), estimate the attributable risk proportion among the exposed (physically inactive). Explain your answer in one sentence. Assume that physical activity is causally related to stroke risk.