Chapter 11 Exercises

Last edit: 5/8/2010

11.1 Descriptive epidemiology of tuberculosis. Search wonder.cdc.gov for up to date tuberculosis information. Then, address these questions:

(A) Time trend. Determine overall tuberculosis morbidity rates over the past 50 years. Plot the rates over time (Y axis = "rate per 100,000 persons per year"; X-axis = year). In plain terms, describe the observed trends and hypothesize things that might have influenced the reported trends (e.g., hypothesize why there was a small increase in the mid-1970s and again in the early-1990s). Search for information that corroborates your hypotheses.
(B) Age-specific rates. Identify TB morbidity rates over time by age. Plot these trends and discuss the findings.
(C) Race-specific rates. Identify and report TB morbidity rates by race. Plot these rates on a single graph. Then, summarize your findings.
(D) Foreign and native incidents. Identify and report TB morbidity rates by country of origin (US Born vs. Foreign born). What percentage of cases are Foreign-born? What country provides the largest number of Foreign-born cases? Present your analysis in graphical form--perhaps in the form of a pie chart. 
(E) Summary. Summarize what you've learned.

11.2 Injuries in farm worker. Read the abstract on this link: McGwin et al. (Am J Epidemiol 2000;152:640-50). Then, answer these questions:

(A) List the exposures addressed by this study. What is the “disease” outcome? 
(B) Explain why experimentation is not possible when addressing this exposure.  
(C) Explain why this study is non-experimental. 
(D) Are data prospective or retrospective?
(E) What measures of association can be estimated from these data? 
(F) Some of the data from the study are shown below. Calculate the RRs of injury in each group using the Caucasian owners as the reference group. Is there a risk associated with race? Is there a risk associated with being a worker? 

Group	Cases	Person-years
Caucasian Owners�	67	2047
Af-Am. Owners	27	821
Af-Am. Workers�	37	359

11.3 Influenza vaccination and primary cardiac arrest. A case-control study examined the relation between influenza vaccination and primary cardiac arrest (PCA) mortality in King County, Washington between from October 1988 to July 1994 (Siscovick et al. 2000). Fatal cases of PCA (n = 315) were identified from paramedic reports. Community controls (n = 549) were identified using a random digit dialing technique. Spouses of cases and controls were interviewed to ascertain who had received influenza vaccination during the prior year. Data are shown here:

	Cases	Controls
Vaccinated	79	176
Not vaccinated	236	373

(A) Explain why this is a case-control study and not a cohort study.
(B) Calculate the odds ratio associated with vaccination. Interpret this finding.
(C) Why were spouses used to ascertain the exposure status of cases? [Hint: This is a study of PCA mortality.]

(D)� Why do you suppose spouses were used to ascertain the exposure status of controls? [Advanced question: please use the class discussion boards to discuss this issue.]

11.4 Case-control study of pancreatic cancer and meat consumption (Norell et al. 1986; Ahlbom & Norell, 1990, p. 52). Data from a case-control study of pancreatic cancer and eating fried or grilled meats at least once per week are shown here. Calculate the odds ratio and discuss the findings. 

	Cases	Controls
E+	53	53
E-	43	85

11.5 Doll & Hill, 1950. Data from a historically important case-control study of smoking and lung cancer are shown below (Doll & Hill, 1950). Calculate the odds ratio and interpret the results. 

	Cases	Controls
Smoker	647	622
Nonsmoker	2	27

11.6 IUDs and infertility.  A study of infertility found prior use of IUDs in 89 of 283 infertile women. In contrast, 640 out of 3833 controls (fertile women) had used IUDs in the past (Cramer et al., 1985; Rosner, 1990, p. 381). Put the data in 2-by-2 table form, determine the odds ratio, and interpret your results.

11.7 OC studies from the 70s. A causal relation between oral contraceptive use and cardiovascular disease was first postulated in the late 1960s. Numerous studies have since confirmed the relationship. Abstracts from two early epidemiologic studies on this topic are shown below. Read these abstracts and answer the questions that follow.

Study 1 (Mann et al., 1975): Mann, J. I., Vessey, M. P., Thorogood, M., & Doll, S. R. Myocardial infarction in young women with special reference to oral contraceptive practice. Br Med J, 2(5965), 241-245; 1975.

Sixty-three women discharged from hospital with a diagnosis of myocardial infarction [heart attack] and 189 control patients were studied. All were under 45 years of age at the time of admission. Current oral contraceptive use, heavy cigarette smoking, treated hypertension and diabetes, pre-eclamptic toxaemia, and obesity were all reported by, and type II hyperlipoproteinaemia was found more often in, patients with myocardial infarction than their controls. The relationship between myocardial infarction and oral contraceptives could not be explained in terms of an association between the use of these preparations and the other factors. The combined effect of the risk factors was clearly synergistic.

Study 2 (Royal College, 1977): Royal College of General Practitioners' Oral Contraception Study.� Mortality among Oral-Contraceptive Users. Lancet Oct 8;2(8041):727 - 31; 1977.
In a large prospective study carried out in the United Kingdom, the death-rate from diseases of the circulatory system in women who had used oral contraceptives was five times that of [women] who had never used them; and the death-rate in those who had taken the pill continuously for 5 years or more was ten times that of the [non-users]. The excess deaths in oral-contraceptive users were due to a wide range of vascular conditions. The total mortality-rate in women who had ever used the pill was increased by 40%, and this was due to an increase in deaths from circulatory diseases of 1 per 5000 ever-users per year. The excess was substantially greater than the death-rate from complications of pregnancy in the [non-users], and was double the death-rate from accidents. The excess mortality-rate increased with age, cigarette smoking, and duration of oral contraceptive use.

(A) What makes the first study a case-control study?

(B) What makes the second study a cohort study?
(C) The hospital served as the source of cases and controls in Mann et al. (1977). This allows for a "captive audience," accurate diagnoses, and access to medical information. Recall that cases and controls in case-control studies should by a random sample cases and non-cases from the population (in this case, the “catchment area” of the hospital). What potential problems do you see in using hospitalized cases and controls to identify study subjects?  
(D) The abstract for Royal College (1977) fails to report the actual mortality rates in the various group but notes that the "total mortality-rate in women who had ever used the pill was increased by 40%." Based on this statement, what is the rate ratio associated with oral contraceptive use? 
(E) The Royal College study (1977) also states that the increase "was due to an increase in deaths from circulatory disease of 1 per 5000 ever-users per year." Does this statistic represent a  rate ratio, rate difference, or attributable fraction? Explain.
(F) Comparison of case-control and cohort designs:

(1) Which study allowed for a smaller total sample size?
(2) Assuming the cohort study was not retrospective, which study took less time to complete?
  (3) Can case-control studies estimate the incidence of disease? Explain.
(4) Which study, Mann et al. or the Royal College of Practitioners, was probably less prone to recall bias?
(5) Which was less prone to selection bias?
(6) Which was less prone to loss to follow-up?
(7) Why do you think the case-control study done before the cohort study?

11.8 Breast Cancer in Canada (descriptive epi). Complete the "Breast Cancer in Canada" case study posted on http://www.math.mcmaster.ca/peter/sora/case_studies_99/breastcancer.html .

11.9 Descriptive epidemiology. Descriptive epidemiologic studies use routinely collected data to explore patterns of disease by person, place, and time factors. Select a specific type of cancer and the explore its occurrence using http://wonder.cdc.gov/. Complete the following comparisons: 

(A) Regional comparisons. Does the risk of dying from the outcome differ by state or region (after adjusting for age and other population differences)?
(B) Temporal trends. What are the trends over time? In which regions are the rates increasing? Where are they decreasing?
(C) Age-specific trends. Compare age-specific trends. Are risks increasing for some age groups but not for others?
(D) Age-and cohort effects. Attempt to discriminate between trends due to changes in risk factors and changes in diagnostic practices and registration procedures. Also, look for birth cohort effects.

11.10 Pedicure-associated furunculosis. A case-control study was initiated to examine the causes of an outbreak of pedicure-associated Mycobacteria furunculosis at nail salons in Santa Clara County California during the summer 2004. For this analysis, case salons were defined as those salons that were culture positive for acid-fat bacteria from environmental samples. Control salons were recruited randomly from a list of licensed nail salons that performed pedicures and were confirmed to have foot-spas with no Mycobacterium isolated. The sampling frame for control salons was from the Board of Barbering and Cosmetology licensed agents located in Santa Clara County. Three different levels of cleaning were considered as potential risk factors:� There are:

(A) Lack of adequate cleaning at the end of the day

(B) Lack of adequate cleaning after every client

(C) Lack of extensive weekly cleaning

2-by-2 cross-tabulations are shown for each risk factor below. Calculate the odds ratios associated with each risk factor. Interpret each finding.

	Cases	Controls
Risk Factor (A) +	8	1
Risk Factor (A) −	10	10
Total	18	11

	Cases	Controls
Risk Factor (B) +	12	3
Risk Factor (B) −	5	8
Total	17	11

	Cases	Controls
Risk Factor (C) +	5	1
Risk Factor (C) −	11	9
Total	16	10

11.11 Nail salon outbreak. In the outbreak of M. furunculosis associated with pedicures at nail salons in Northern California, the odds ratio for having a positive environmental cultures associated with incorrect end-of-day cleaning was 8.00.

(A) Based on this odds ratio, what is the attributable fraction among the exposed case salons?

(B) How do you interpret the above attributable fraction?

11.12 Case-control study of ZDV and HIV following needle sticks. �Healthcare workers will occasionally be exposed to HIV through needle sticks and other accidental punctures with sharp objects. A case-control study in the United Kingdom was conducted to determine the effects of administering zidovudine (ZDV) to healthcare workers following needle stick injuries. The study included 31 healthcare workers who became HIV positive (cases), of which 9 had received ZDV. A group of 679 HIV-negative healthcare workers who also sustained needle sticks served as controls.� Of these controls, 247 had received ZDV.

(A) Create a 2-by-2 table to illustrate the data.

	Cases (HIV+)	Controls (HIV −)
ZDV+
ZDV−
	31	679

(B) Calculate the odds ratio.

(C) Is there evidence that ZDV decreases the risk of HIV following needle stick injuries? If so, how much does the ZDV cut down on risk?