Epi Modules Review

The underreporting by the case group suggests that exposure may have been misclassified more often in the case group than in the control group; this is differential misclassification

In a case-control study of the relationship between intravenous drug use and HIV infection, the investigators discovered after the study that the case group had tended to underreport their drug use due to fears arising from previous convictions for drug-related violations. Underreporting was not a problem in the control group, however, which was not under medical supervision at the time. Information bias is likely to influence the results obtained in this study because:

Information bias

In a case-control study of the relationship between intravenous drug use and HIV infection, the investigators discovered after the study that the case group had tended to underreport their drug use due to fears arising from previous convictions for drug-related violations. Underreporting was not a problem in the control group, however, which was not under medical supervision at the time. The above scenario is an example of:

will tend to underestimate the true measure of association [be negatively biased] since the case group tended to underreport their drug use

In a case-control study of the relationship between intravenous drug use and HIV infection, the investigators discovered after the study that the case group had tended to underreport their drug use due to fears arising from previous convictions for drug-related violations. Underreporting was not a problem in the control group, however, which was not under medical supervision at the time. The measure of association obtained in this study:

Negative bias

In a large hospital-based case-control study of smoking and lung cancer, controls consisted of patients with noncancerous chronic pulmonary diseases, such as chronic bronchitis and emphysema. The investigators found that the association between smoking and lung cancer was relatively weak [OR = 1.1, p < 0.05]. This was surprising as later studies, including cohort studies, reported odds ratios and risk ratios > 1 for smoking and lung cancer. [select all that apply]

Negative bias [Negative Bias CORRECT: The likely error in this problem was due to the fact that the control group probably over-represents smokers (current or former) compared to all possible controls in the hypothetical source population. Because smoking is related to lung cancer, this would have the effect of reducing the magnitude of the measure of association between smoking and lung cancer since cases and controls would tend to be more alike with respect to their smoking history. The result is a negative bias. It would have been more appropriate to have selected controls from diagnoses unrelated to smoking. Bias away from the null value Bias toward the null value CORRECT: The likely error in this problem was due to the fact that the control group probably over-represents smokers (current or former) compared to all possible controls in the hypothetical source population. Because smoking is related to lung cancer, this would have the effect of reducing the magnitude of the measure of association between smoking and lung cancer since cases and controls would tend to be more alike with respect to their smoking history. The result is a negative bias. It would have been more appropriate to have selected controls from diagnoses unrelated to smoking.]

In a large hospital-based case-control study of smoking and lung cancer, controls consisted of patients with noncancerous chronic pulmonary diseases, such as chronic bronchitis and emphysema. The investigators found that the association between smoking and lung cancer was relatively weak [OR = 1.1, p < 0.05]. This was surprising as later studies, including cohort studies, reported odds ratios and risk ratios > 1 for smoking and lung cancer. [select all that apply]

True [This is an ecological study because the unit of analysis is the group [i.e., one of 12 geographic regions in Sweden]]

In order to determine if vitamin D [the "sunshine" vitamin] is associated with reduced levels of ovarian cancer, an epidemiologist identified 12 geographic regions in Sweden and obtained data on the average annual number of days of sunlight and ovarian cancer rates for each region. The findings indicated that those regions with the highest average days of sunlight per year had significantly lower rates of ovarian cancer than those regions with the fewest average days of sunlight per year. The study described above is an ecologic study:

Anyone randomized into one of the study groups must be included in the final analysis in the group to which they were originally randomized, regardless of whether they adhered to the protocol or not

Intention-to-Treat analysis of a clinical trial means which of the following?

[35/300]-[15/700]

Recall the cohort study described earlier: A cohort study of obesity and diabetes was conducted in a small, remote town with little in-migration and out-migration. The study investigators recruited 1,000 residents into the study and followed them on a yearly basis for 10 years. At baseline, 300 participants were overweight or obese [defined as BMI ≥ 25] and 700 had normal BMIs [< 25]. Among those who were obese, 35 developed diabetes whereas 15 of those with normal BMI developed diabetes. The attributable risk for diabetes in those who have high BMI is:

Random error

The authors of a cross-sectional study hypothesized that lack of regular exercise is associated with obesity in children. Their study of 12 children in Michigan, however, failed to show a statistically significant association between exercise habits and obesity [OR = 1.9, p = 0.11].

Random error The small sample size of only 12 children suggests that low power may have led to a type II error, especially since this association is likely to exist. A type II error is a type of random error]

The authors of a cross-sectional study hypothesized that lack of regular exercise is associated with obesity in children. Their study of 12 children in Michigan, however, failed to show a statistically significant association between exercise habits and obesity [OR = 1.9, p = 0.11].

Rate ratio [incidence density]; Disease odds ratio; Risk ratio [cumulative incidence]

The following measure(s) of association are observed, or can be calculated from data obtained from a cohort study [select all that apply]:

Exposure odds ratio

The following measure(s) of association are observed, or can be calculated from data obtained in a case-control study [select all that apply]:

Proportion attributable risk [By dividing the Attributable Risk by the incidence in the exposed, we can know what proportion of incident cases in the exposed group were due to the exposure]

The formula shown below represents which of the following measures of impact:

The degenerated samples are likely to have resulted in some incorrect classification of exposure status in both the cases and controls

The investigators of a prospective cohort study collected blood samples from the participants at the beginning of the study and froze them for later analysis. A subsequent nested case-control study was conducted to determine if blood levels of certain hormones were associated with Alzheimer's disease. The previously stored blood samples were analyzed for the cases and controls. It was later determined that the samples had degenerated during the storage period because of a failure to maintain consistent temperatures. The above scenario is an example of information bias because:

The misclassification is likely to represent nondifferential misclassification since presumably all the samples were affected uniformly

The investigators of a prospective cohort study collected blood samples from the participants at the beginning of the study and froze them for later analysis. A subsequent nested case-control study was conducted to determine if blood levels of certain hormones were associated with Alzheimer's disease. The previously stored blood samples were analyzed for the cases and controls. It was later determined that the samples had degenerated during the storage period because of a failure to maintain consistent temperatures. Select the correct statement:

[500] [3,000] / [500] [1,000] = 3.0 [this is the ad/bc "cross-products" ratio]

The odds ratio quantifying the relationship between educational attainment and unintentional injury in this sample is calculated as:

False [this is a cross-sectional study where a sample of individuals is recruited and surveyed about their individual diabetes status]

The prevalence of type 2 diabetes was determined from a randomly selected sample of 3,000 adults over 50 years of age in a large city in the Southwest. Based on clinical examinations and testing, the prevalence of diabetes was 7.5% in men and 6.2% in women. The prevalence of diabetes increased with age more among the women than the men. Also, the rates in both men and women were higher than they had been in past years. The study described above is an ecologic study:

[35/300-15/700]/[35/300]

The proportion attributable risk for diabetes in those who have high BMI is:

To avoid selection bias, controls should be selected from the same population that gave rise to the cases

The selection of controls [i.e. the comparison group] in a case-control study is guided by which of the following principles? [Select all that apply.]

It is easier to break the blind of smaller trials and figure out which treatment patients are getting; The statistical power of the study may be limited if there aren't enough subjects who develop the primary outcome that is being measured; Confounding factors may not be equally distributed between groups

Very small randomized clinical trials may have which of the following problems? [Select all that apply]

Blinding means that the subjects and/or investigators do not know which treatment group the subject is in. The purpose is to prevent bias in assessing the outcome

What is "blinding" and what is its purpose?

The degree to which study subjects adhere to an assigned treatment protocol

What is meant by "compliance" in a randomized clinical trial?

a method of allocating treatment such that each subject has an equal chance of receiving any of the possible treatments

What is meant by "randomization" in an RCT?

If compliance is poor, the two groups will become more alike and the ability to detect true differences in outcome is diminished, i.e., the power of the study is reduced

What problem results when compliance of subjects is poor in an intervention study?

They tend to be less expensive and more efficient than prospective cohort studies; They are more efficient for studying rare diseases; They are more efficient for studying diseases that have a long latency period [i.e., a long time between exposure and manifestation of disease]

Which of the following are advantages to case-control studies? [Select all that apply].

Enroll motivated and knowledgeable subjects who lead fairly organized lives; During enrollment take a careful medical history to try to identify individuals who would have a difficult time complying, and exclude these people from the study; Accurately explain all study procedures and requirements during the enrollment and informed consent process, so participants know what the study entails and what will be required of them

Which of the following are strategies to increase compliance in a clinical trial? [Select all that apply]

The odds ratio calculated in a case-control study yields a fairly good approximation to the relative risk provided the outcome is uncommon

Which of the following statements correctly describes case-control studies?

The purpose of blinding is to reduce confounding

Which statement about blinding in an intervention study is NOT correct?

In a double blinded study, neither the subject nor the investigators know which treatment the subject is receiving; Blinding can be accomplished by using a placebo; The purpose of blinding is to reduce bias in determining the outcome

Which statements about blinding in an intervention study are correct?

69/209 [this is a comparison of two probabilities: first, the probability of TB [being a case] among smokers [the exposed - a/a+b] divided by the probability of no TB [being a control] among smokers [the exposed - b/a+b]

A case-control study was conducted at the Seattle/King County Tuberculosis Clinic from 1988 through 1990 (Buskin et al. AJPH, 1994) to examine associations between smoking status and diagnosis with active tuberculosis (TB). Among those screened, 69 individuals who reported currently smoking were diagnosed as TB cases whereas 209 current smokers who did not have active TB served as controls. Among those who reported never smoking, 48 individuals were diagnosed with active TB were included in the case group while 245 were TB-free and included in the control group. What is the odds of TB among current smokers?

48/245 [this is a comparison of two probabilities: first, the probability of TB [being a case] among non-smokers [the unexposed - c/c+d] divided by the probability of no TB [being a control] among non-smokers [the unexposed - d/[c+d] ]

A case-control study was conducted at the Seattle/King County Tuberculosis Clinic from 1988 through 1990 (Buskin et al. AJPH, 1994) to examine associations between smoking status and diagnosis with active tuberculosis (TB). Among those screened, 69 individuals who reported currently smoking were diagnosed as TB cases whereas 209 current smokers who did not have active TB served as controls. Among those who reported never smoking, 48 individuals were diagnosed with active TB were included in the case group while 245 were TB-free and included in the control group. What is the odds of TB among non-smokers?

[69 x 245] / [209 x 48] [The odds ratio is a comparison of the odds of TB among smokers to the odds of TB among non-smokers]

A case-control study was conducted at the Seattle/King County Tuberculosis Clinic from 1988 through 1990 (Buskin et al. AJPH, 1994) to examine associations between smoking status and diagnosis with active tuberculosis (TB). Among those screened, 69 individuals who reported currently smoking were diagnosed as TB cases whereas 209 current smokers who did not have active TB served as controls. Among those who reported never smoking, 48 individuals were diagnosed with active TB were included in the case group while 245 were TB-free and included in the control group. Which of the following describes the odds ratio for TB and smoking?

The odds of active TB is 1.68 times higher among those who smoke compared to those who do not smoke [This is a comparison of the odds of being a case [having TB] among those who are exposed [smoke] to the odds of being a case among those who are not exposed [don't smoke] ]

A case-control study was conducted at the Seattle/King County Tuberculosis Clinic from 1988 through 1990 (Buskin et al. AJPH, 1994) to examine associations between smoking status and diagnosis with active tuberculosis (TB). Among those screened, 69 individuals who reported currently smoking were diagnosed as TB cases whereas 209 current smokers who did not have active TB served as controls. Among those who reported never smoking, 48 individuals were diagnosed with active TB were included in the case group while 245 were TB-free and included in the control group. Which of the following statements correctly interprets the association between active TB and smoking as calculated earlier?

OR = 2.0

A case-control study was conducted to assess the relationship between prior urothelial carcinomas and bladder cancer. A total of 1,500 cases and 3,000 controls were enrolled in the study. Amongst the cases, 647 had used artificial sweeteners in the past, while 853 had never used artificial sweeteners. Among the controls, 825 had used sweeteners and 2,175 had not. Compute the appropriate measure of association.

Bias away from the null value; Positive bias

A case-control study was designed to test whether persons exposed to certain types of pesticides during early childhood were more likely to develop neurological problems in later adulthood. The investigators obtained an OR = 4.6 from this study. However, upon further investigation, it appears that, in reality, there is no relationship between early childhood pesticide exposure neurological problems in adulthood [OR = 1]. Which of the following terms correctly identifies the direction of bias observed in the case-control study compared to the relationship between early childhood pesticide exposure and neurological problems in adulthood as observed in reality? [select all that apply]:

Bias away from the null value; Positive bias [Positive Bias CORRECT: The statistically significant result was probably influenced to some degree by recall bias, a type of information bias. The cases are more likely to have remembered past pesticide exposures than the controls because of their interest in understanding the causes of their disorders. This would have produced a positive bias in the findings; Negative Bias: INCORRECT Bias away from the null value CORRECT: The statistically significant result was probably influenced to some degree by recall bias, a type of information bias. The cases are more likely to have remembered past pesticide exposures than the controls because of their interest in understanding the causes of their disorders. This would have produced a positive bias in the findings. Bias toward the null value INCORRECT]

A case-control study was designed to test whether persons exposed to certain types of pesticides during early childhood were more likely to develop neurological problems in later adulthood. The investigators obtained an OR = 4.6 from this study. However, upon further investigation, it appears that, in reality, there is no relationship between early childhood pesticide exposure neurological problems in adulthood [OR = 1]. Which of the following terms correctly identifies the direction of bias observed in the case-control study compared to the relationship between early childhood pesticide exposure and neurological problems in adulthood as observed in reality? [select all that apply]:

[35/300]/[15/700]

A cohort study of obesity and diabetes was conducted in a small, remote town with little in-migration and out-migration. The study investigators recruited 1,000 residents into the study and followed them on a yearly basis for 10 years. At baseline, 300 participants were overweight or obese [defined as BMI ≥ 25] and 700 had normal BMIs [< 25]. Among those who were obese, 35 developed diabetes whereas 15 of those with normal BMI developed diabetes. Which of the following is the correct value for the measure of association in the above study?

Risk Ratio [the Risk Ratio is the appropriate measure of association used to evaluate the strength of an association between an exposure and an outcome in a cohort study]

A cohort study of obesity and diabetes was conducted in a small, remote town with little in-migration and out-migration. The study investigators recruited 1,000 residents into the study and followed them on a yearly basis for 10 years. At baseline, 300 participants were overweight or obese [defined as BMI ≥ 25] and 700 had normal BMIs [< 25]. Among those who were obese, 35 developed diabetes whereas 15 of those with normal BMI developed diabetes. Which of the following measures should be used to determine the strength of the association between obesity and diabetes?

Selection bias

A prospective cohort study followed 8,542 women for 10 years to determine if alcohol consumption increased the risk of breast cancer. No statistically significant association was found. Twelve thousand women had been enrolled in the study at its initiation, but 3,458 had withdrawn during the 10-year period. A comparison of baseline data for those who remained in the study (completed all follow-up visits) and those withdrew from the study revealed that more than 60% of the withdrawals had developed breast cancer and that nearly 84% were regular drinkers. The above scenario is an example of:

The losses tended to be drinkers who developed breast cancer, so if they had been included in the study results, the findings may have changed [i.e., an association between alcohol consumption and breast cancer may have been found]; This study illustrates a high probability selection bias due to loss to follow-up

A prospective cohort study followed 8,542 women for 10 years to determine if alcohol consumption increased the risk of breast cancer. No statistically significant association was found. Twelve thousand women had been enrolled in the study at its initiation, but 3,458 had withdrawn during the 10-year period. A comparison of baseline data for those who remained in the study [completed all follow-up visits] and those withdrew from the study revealed that more than 60% of the withdrawals had developed breast cancer and that nearly 84% were regular drinkers. How might these losses have influenced study results [select all that apply]:

Those who engage in daily physical activity are at 0.6 times the risk of developing obesity as compared to those who do not engage in physical activity [a Risk Ratio compares the risk of experiencing an outcome among those who are exposed to the risk of developing that outcome among those who are unexposed]

A second cohort study is conducted in the same population to examine factors associated with development of obesity in another sample of 1,000 residents. The study findings indicate that compared to individuals who do not engage in any physical activity, those who do engage in daily physical activity have a Risk Ratio = 0.60 with respect to development of obesity. Which of the following statements is a correct interpretation of this Risk Ratio?

True

An advantage of using hospital-based controls is that they are more likely to cooperate and are likely to remember past exposures with the same accuracy as the cases.

This means that the case group [those with migraines] are more likely than the control group [those in the source population without migraines] to be using diet pills; hence, the greater the probability of finding an association between migraines and the use of diet pills in this sample

An epidemiologist examines the association between the use of diet pills and migraine headaches using a case-control study design. Her participants are individuals seeking outpatient care at a large community hospital. She finds that those using diet pills are more likely to complain of migraines, and also discovers that those who have migraine headaches and use diet pills are more likely to be referred to the same community hospital for outpatient diagnostic testing than those who have migraines but are not using diet pills. What are the implications of this type of referral pattern?

Prevalence = [1,500 / 5,000] x 100 = 30.0 injuries per 100 population [There were 30.0 unintentional injuries in the home for every 100 men and women in the sample. In other words, the overall prevalence was 30.0%]

An investigator was interested in examining whether educational attainment was associated with unintentional injuries among adults. The investigators selected a random sample of 5,000 men and women, 21 years of age and older in Portland, Oregon. The results of this cross-sectional study were as follows: 10% of the sample had high educational attainment (defined as a college degree or higher) and unintentional injuries in the previous 12 months 10% of the sample had high educational attainment but no unintentional injuries in the previous 12 months 20% of the sample had low educational attainment (defined as having less than a college degree) and unintentional injuries in the previous 12 months 60% of the sample had low educational attainment but no unintentional injuries in the previous 12 months. What is the overall prevalence of unintentional injuries in this sample of adult men and women residing in Portland, OR?

0.54% [48 + 12/1,000 + 10,000 = 0.54%]

As part of pre-natal screening tests health care provider, will offer a quad marker screening test to expectant mothers between 15 - 20 weeks of pregnancy. To conduct the quad marker screen, a blood sample from the mother is taken and tested for the presence of four markers: Alpha-fetoprotein (AFP), Unconjugated Estriol (UE), Human Chorionic Gonadotropin (hCG), and Inhibin-A. The presence and level of these markers provides information on problems in the development of the fetus's brain and spinal cord (neural tube defects) as well as genetic disorders such as Down syndrome. The following data were collected to determine the validity of this test with regard to neural tube defects: Neural Tube Defects Yes No Quad Screen Result Positive 48 952 Negative 12 9,988 What is the prevalence of neural tube defects in this population of pregnant women?

Among those with a positive quad screen test result, it is the probability of having a child with a neural tube defect [PPV is the proportion of positive test results that are true positives]

As part of pre-natal screening tests health care provider, will offer a quad marker screening test to expectant mothers between 15 - 20 weeks of pregnancy. To conduct the quad marker screen, a blood sample from the mother is taken and tested for the presence of four markers: Alpha-fetoprotein (AFP), Unconjugated Estriol (UE), Human Chorionic Gonadotropin (hCG), and Inhibin-A. The presence and level of these markers provides information on problems in the development of the fetus's brain and spinal cord (neural tube defects) as well as genetic disorders such as Down syndrome. The following data were collected to determine the validity of this test with regard to neural tube defects: Neural Tube Defects Yes No Quad Screen Result Positive 48 952 Negative 12 9,988 Based on the value for the positive predictive value (PPV) of the quad marker screening test calculated above, what does the PPV of this test mean?

Among those with a neural tube defect, it is the probability of having a positive quad marker screening test [Sensitivity is the probability that a screening test correctly identifies disease among people who have the disease]

As part of pre-natal screening tests health care provider, will offer a quad marker screening test to expectant mothers between 15 - 20 weeks of pregnancy. To conduct the quad marker screen, a blood sample from the mother is taken and tested for the presence of four markers: Alpha-fetoprotein (AFP), Unconjugated Estriol (UE), Human Chorionic Gonadotropin (hCG), and Inhibin-A. The presence and level of these markers provides information on problems in the development of the fetus's brain and spinal cord (neural tube defects) as well as genetic disorders such as Down syndrome. The following data were collected to determine the validity of this test with regard to neural tube defects: Neural Tube Defects Yes No Quad Screen Result Positive 48 952 Negative 12 9,988 Based on the value for the sensitivity of the quad marker screening test calculated above, what does the sensitivity of this test mean?

Among those without a neural tube defect, it is the probability of having a negative quad marker screening test [Sp is the probability that a screening test correctly identifies non-disease status among people who do NOT the disease]

As part of pre-natal screening tests health care provider, will offer a quad marker screening test to expectant mothers between 15 - 20 weeks of pregnancy. To conduct the quad marker screen, a blood sample from the mother is taken and tested for the presence of four markers: Alpha-fetoprotein (AFP), Unconjugated Estriol (UE), Human Chorionic Gonadotropin (hCG), and Inhibin-A. The presence and level of these markers provides information on problems in the development of the fetus's brain and spinal cord (neural tube defects) as well as genetic disorders such as Down syndrome. The following data were collected to determine the validity of this test with regard to neural tube defects: Neural Tube Defects Yes No Quad Screen Result Positive 48 952 Negative 12 9,988 Based on the value for the specificity of the quad marker screening test calculated above, what does the specificity of this test mean?

4.8% [PPV = true positive test results/all positive test results: a/a+b = 48/48 + 952 = 48/1000 = 4.8%]

As part of pre-natal screening tests health care provider, will offer a quad marker screening test to expectant mothers between 15 - 20 weeks of pregnancy. To conduct the quad marker screen, a blood sample from the mother is taken and tested for the presence of four markers: Alpha-fetoprotein (AFP), Unconjugated Estriol (UE), Human Chorionic Gonadotropin (hCG), and Inhibin-A. The presence and level of these markers provides information on problems in the development of the fetus's brain and spinal cord (neural tube defects) as well as genetic disorders such as Down syndrome. The following data were collected to determine the validity of this test with regard to neural tube defects: Neural Tube Defects Yes No Quad Screen Result Positive 48 952 Negative 12 9,988 For the problem above, what is the positive predictive value of the screening test?

The PPV would increase

As part of pre-natal screening tests health care provider, will offer a quad marker screening test to expectant mothers between 15 - 20 weeks of pregnancy. To conduct the quad marker screen, a blood sample from the mother is taken and tested for the presence of four markers: Alpha-fetoprotein (AFP), Unconjugated Estriol (UE), Human Chorionic Gonadotropin (hCG), and Inhibin-A. The presence and level of these markers provides information on problems in the development of the fetus's brain and spinal cord (neural tube defects) as well as genetic disorders such as Down syndrome. The following data were collected to determine the validity of this test with regard to neural tube defects: Neural Tube Defects Yes No Quad Screen Result Positive 48 952 Negative 12 9,988 If the prevalence of neural tube defects in this population were to increase, how would this impact the PPV of the quad marker screening test?

80% [Sn = a/a+c = 48/48+12 = 80%]

As part of pre-natal screening tests health care provider, will offer a quad marker screening test to expectant mothers between 15 - 20 weeks of pregnancy. To conduct the quad marker screen, a blood sample from the mother is taken and tested for the presence of four markers: Alpha-fetoprotein (AFP), Unconjugated Estriol (UE), Human Chorionic Gonadotropin (hCG), and Inhibin-A. The presence and level of these markers provides information on problems in the development of the fetus's brain and spinal cord (neural tube defects) as well as genetic disorders such as Down syndrome. The following data were collected to determine the validity of this test with regard to neural tube defects: Neural Tube Defects Yes No Quad Screen Result Positive 48 952 Negative 12 9,988 What is the sensitivity of the quad marker screening test for neural tube defects?

91.2% [Sp = d/c + d = 9,988/10,940 = 91.2%]

As part of pre-natal screening tests health care provider, will offer a quad marker screening test to expectant mothers between 15 - 20 weeks of pregnancy. To conduct the quad marker screen, a blood sample from the mother is taken and tested for the presence of four markers: Alpha-fetoprotein (AFP), Unconjugated Estriol (UE), Human Chorionic Gonadotropin (hCG), and Inhibin-A. The presence and level of these markers provides information on problems in the development of the fetus's brain and spinal cord (neural tube defects) as well as genetic disorders such as Down syndrome. The following data were collected to determine the validity of this test with regard to neural tube defects: Neural Tube Defects Yes No Quad Screen Result Positive 48 952 Negative 12 9,988 What is the specificity of the quad marker screening test for neural tube defects?

False [In an ecologic study, we cannot know if those individuals who experienced an outcome were more or less likely to also have experienced the exposure of interest. Making this type of claim would be considered an ecologic fallacy]

Based on the study described in question 2, one can surmise that those who developed ovarian cancer were more likely to have lower levels of vitamin D.

10 years

Based on the timeline shown above, how many years did the screening test add to this person's life?

30 years [Breast cancer is identified at age 45 via screening and death does not occur until age 75 [30 years later] in the early screening scenario.]

Based on the timeline shown above, what is the amount of survival time following early detection of breast cancer, in other words, what is the length of the lead time?

15 years [the DPCP is the interval during which the disease is still in a pre-clinical or sub-clinical phase but there has been sufficient disease process so that the disease can be detected via screening]

Based on the timeline shown above, what is the length of the detectable preclinical phase [DPCP]?

None of the above

n epidemiologist examines the association between the use of diet pills and migraine headaches using a case-control study design. Her participants are individuals seeking outpatient care at a large community hospital. She finds that those using diet pills are more likely to complain of migraines, and also discovers that those who have migraine headaches and use diet pills are more likely to be referred to the same community hospital for outpatient diagnostic testing than those who have migraines but are not using diet pills. The above scenario is an example of selection bias because of which of the following reasons: