Measures Of Association, Statistical Tests, & Interpretation
Directions: Write the information requested in each column. Please refer to the Variables Supplement found in the Resources. Each scenario is 14 points each, total 70 points. Question F is worth 5 points for a total of 75 points.
Select from these study designs, using each only once:
Case control study
Prospective cohort study
Retrospective cohort study
Cross-sectional study
Intervention trial
Select from these measures of association and tests of significance, some of which are used more than once:
Chi-square
Chi-square w/o Yate’s Correction
Difference between the means
Chi-square Fishers Exact Test
Odds ratio
p-value
Relative risk
t-test
Scenario A: A team of researchers set out to study the risk factors for pancreatic cancer. From hospital records, they assembled a group of 50 patients who were either still living or who had a next of kin identified and willing to answer questions about medical history, lifestyle, and prior experiences of the patient. They assembled another group of 50 patients from the same hospital who were admitted for other causes and did not have cancer of any type. The investigators conducted a review of all available records of these patients, then interviewed them or their next of kin using a standardized format, either in person or by telephone. They focused on ten possible risk factors which could either be answered as yes/no or rated on an ordinal scale. In this study, 22 of the 50 pancreatic cancer patients were recorded as having at least a 20 pack-year history of cigarette smoking, while 7 of the 50 in the other group of patients had that history.
1. What type of study is this and what attributes make it so? (See Week 2 PPT):
Case-control study
2. What is the independent variable in this scenario, and what type is it? What is the dependent variable in this scenario, and what type is it?
Independent= 20 pack a year smoking
type=
Dependent=pancreatic cancer
type=
3. A null hypothesis states that there is no effect, no relationship, or no difference between the variables being tested. What might be a null hypothesis for this study?
4. Odds Ratio is the most appropriate measure of association to evaluate the relationship of each risk factor for the occurrence of pancreatic cancer and measures 4.47. Use the table and link below to show the data. What do the results mean?
The Odds Ratio is calculated as (a/c) / (b/d).
In this scenario, which group is ___ times more likely to develop pancreatic CA?
What types of variables can be tested by OR?
# of Ills
# of Wells
Total
# of Exposed
A =
B =
A + B = 50
# of Unexposed
C =
D =
C + D = 50
Total
A + C = 50
B + D = 50
A+B+C+D = 100
§ OR = 1 suggests that there is no difference between the groups; i.e. there is no association between the suggested exposure and the outcome (being ill)
§ OR > 1 suggests that the odds of exposure are positively associated with the adverse outcome compared to the odds of not being exposed
§ OR < 1 suggests that the odds of exposure are negatively associated with the adverse outcomes compared to the odds of not being exposed. Potentially, there could be a protective effect
5. What is the most appropriate test of significance for this scenario? (See Week 2 PPT)
Chi-square test for Independence w/o Yate’s correction
6. Calculate the chi-square test result using this on-line calculator. What is the result?
7. What is the interpretation of the test result and what do the results mean?
The chi-squared statistic is a single number that tells you how much difference exists between your observed counts and the counts you would expect if there were no relationship at all in the population.
Scenario B: Another team of researchers set out to test a new medication developed to prevent the onset of diabetes in patients with a strong family history of diabetes. They assembled a group of 200 patients and randomized them into two equal groups, one to receive a placebo and the other to receive the medication. They followed them each year for five years and ascertained the occurrence of diabetes using a glucose tolerance test. At the end of the trial, 11 of 100 patients in the placebo groups had developed diabetes mellitus, and 8 of 100 patients in the medication group had developed diabetes mellitus.
1. What type of study is this and what attributes make it so? (See week 2 PPT)
Intervention trial
2. What is the independent variable in this scenario, and what type is it? What is the dependent variable in this scenario, and what type is it?
3. Please state a null hypothesis.
4. What is the most appropriate measure of association for this scenario? (see week 2 PPT)
Relative risk
5. What is the most appropriate test of significance for this scenario? (See week 2 PPT.) What do the results mean?
Chi square
6. Fill in the online Chi-square contingency table. Set it up just as you did earlier. What is the result? Chi-square is again the most appropriate test for significance. Why?
7. What do the results mean in terms of the null?
Relative Risk is very similar to Odds Ratio, however, RR is calculated by using percentages, whereas Odds Ratio is calculated by using the ratio of odds. Relative Risk values are greater than or equal to zero. A value of 1 indicates a neutral result meaning the probability of a variable being present in one group is the same probability of a variable being present in the other group. However, a value of zero indicates that none of the cases in group 1 had the event occur while x number of cases in group 2 had the event occur; or in other words, the numerator was a zero (A = 0) and the denominator was any number greater than zero (B = x, where x > 0).
Scenario C: A team of health sciences students decided on a project that would give them experience with their statistical analysis skills as well as some practice in taking vital signs. They assembled a group of 40 undergraduate students, 20 science majors and 20 majors in other fields of study, brought them to a makeshift clinic in the science building and measured their blood pressures with a hand-held BP cuff. The results were as follows:
The science majors’ SBPs were: 100, 110, 116, 120, 122, 128, 128, 128, 130, 132, 132, 134, 140, 144, 146, 146, 150, 152, 154, 158.
The other majors’ SBPs were: 112, 118, 120, 126, 130, 130, 132, 134, 140, 142, 144, 144, 148, 150, 158, 160, 170, 172, 176, 190
1. What type of study is this and what attributes make it so? (See Week 2 PPT)
Cross-sectional study
2. What is the independent variable in this scenario, and what type is it? What is the dependent variable in this scenario, and what type is it?
3. Please state a null hypothesis.
4. What is the most appropriate measure of association for this scenario (see week 2 PPT)? Calculate that measure of association.
Significance of the difference between the means
5. What is the most appropriate statistical test of significance of this measure of association. Why?
Independent t-test
6. Calculate the independent t-test result using the on-line calculator (Formula) What are the results?
7. What do the results mean and how do we state our conclusion regarding the null hypothesis?
Scenario D: Another team of investigators reviewed medical records to determine which military personnel among several basic training units had received a previous type of meningitis vaccine during training two decades ago. Two study groups were formed, those who had not received the vaccine and those who had. These were followed forward by record review throughout the duration of their active military service, and if they had a record of care through the VA system, these records were searched as well. The outcome of interest was the occurrence of any of a number of autoimmune diseases that were thought to have a possible association with the vaccine. In this study, of a vaccine group of 70, 4 individuals were found to have been diagnosed with lupus at some subsequent point in time. In the non-vaccine group of 105, only 2 individuals had that diagnosis
1. What type of study is this and what attributes makes it so? (See week 2 PPT):
Retrospective cohort study
2. What is the independent variable in this scenario, and what type is it? What is the dependent variable in this scenario, and what type is it?
3. State a null hypothesis.
4. What is the most appropriate measure of association to compare the rates of occurrence of autoimmune disease between the two groups and Why? Calculate that measure of association.
Relative risk
5. What is the most appropriate statistical test of significance of this measure of association and Why? Calculate the test result using the on-line calculator. Which test option should you use? What is the result?
Chi square, fisher’s exact test
6. Using the same on-line calculator as for problems #1 and #2, perform this test. What are the 2-tailed p-value results?
7. What do the results mean regarding the null hypothesis?
Scenario E: Yet another team of investigators embarked on a long-term study to assess the impact of low versus high levels of blood cholesterol in Native Americans on subsequent occurrence of myocardial infarction. They recruited 100 patients from three Indian Health Service hospital systems, and divided them into two groups based on cholesterol LDL/HDL ratio (high ratio is bad) – 42 in the high group and 58 in the low group. By the end of the fifteen-year follow-up period, 16 in the high group and 7 in the low group had experienced a myocardial infarction.
1. What type of study is this and what makes it so?
Prospective cohort study
2. What is the independent variable in this scenario, and what type is it? What is the dependent variable in this scenario, and what type is it?
3. State a null hypothesis.
4. What is the most appropriate measure of association to compare the rates of myocardial infarction in the two groups and Why? (See Week 2 PPT) Calculate that measure of association. What are the results?
Relative risk
5. Chi square w/o Yates correction is the most appropriate statistical test to test the significance of this measure of association. Why and what are the 2-tailed p-results?
6. What do the results mean in terms of the null hypothesis?
Question F. From your readings, describe:
1. How the study design is selected.
2. How the measure of association is determined.
3. How the measure of significance is determined.
4. How interpretation of the results is accomplished.
5. What you would like to understand better.
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