HP3 Quiz 2 Practice

11

Incidence, Prevalence, Risk and Causation

Which of the following is evidence that an association is cause-and- effect?


  Temporality (cause precedes effect)

  Strength of relationship

  Dose-response relationship

  Reversibility

  All of the above


Recall Bradford Hill criteria for causation

12

Disease X has an incidence rate of 0.2% per annum. Its population prevalence is 1%. Disease X is likely to be:


  Acute tonsillitis

  Acute bacterial pneumonia

  Acute viral upper respiratory tract infection

  Acute urinary tract infection

  Rheumatoid arthritis


For an ‘acute’ self-limiting disease, incidence and prevalence are roughly equal. However, in a chronic disease such as rheumatoid arthritis, prevalence may be markedly greater than incidence, as is the case here.

13

The following information relates to questions 13 to 15 inclusive. The incidence rate of lung cancer in Victorian smokers is 0.3% per annum, while the incidence rate of lung cancer in Victorian nonsmokers is 0.014% per annum. In Victoria, the risk of lung cancer attributable to smoking is:


  0.29% per annum

  0.3% per annum

  0.014% per annum

  0.314% per annum

  None of the above


In Victoria, risk of lung cancer attributable to smoking is: incidence in smokers - incidence in non-smokers = 0.3% - 0.014% = 0.286% per annum).

14

In Victoria, the risk of lung cancer in smokers relative to nonsmokers is:


  0.3 per annum

  0.014 per annum

  21.4 per annum

  0.05 per annum

  None of the above


In Victoria, the risk of lung cancer in smokers relative to non-smokers is: incidence in smokers / incidence in non-smokers = 0.3% / 0.014% = 21.4. This means that over one year, smokers have 21.4 times the risk of lung cancer relative to non-smokers.

15

Assuming the absolute annual risk of lung cancer in Victorian smokers remains constant over time, what is the absolute risk of lung cancer in a Victorian after 10 years of smoking?


  0.3%

  3%

  30%

  6%

  None of the above


Recall that the absolute risk of lung cancer in Victorian smokers is 0.3% per annum. Therefore after 10 years of smoking, the absolute risk of lung cancer would be 0.3% X 10 = 3%. After 20 years, it would be 0.3% X 20 = 6%. Note: these absolute risks are ‘crude’ in that they don’t take into account the number of cigarettes smoked over 1, 10 or 20 years.

16

Association / correlation

The following information and Figure 6 relate to question 16.

Low-density lipoprotein cholesterol (LDL-C) and triglyceride (TG) level were measured in 100 patients with ischemic heart disease. For each patient the LDL-C level was plotted against TG level in a scatter plot.

Figure 6 - scatter plot of LDL-C level versus TG level for 100 patients. r denotes Pearson’s correlation coefficient:



Regarding the information presented in Figure 6, which of the following statements is correct?


  There is a relatively strong linear correlation between LDL-C and TG level

  As LDL-C increases, so does TG level

  81% of the variability in LDL-C relative to TG is explained by a line of ‘best fit’ through the dots

  All of the above

  None of the above


A is correct because r = 0.9 (i.e. very close to 1); B is correct because r is positive and also looking at graph, as LDL-C increases, so does TG; C is correct because r2 = 0.92 = 0.81 = 81%.

More explanation about the correlation coefficient (r) and the coefficient of determination (r squared) follow this in the revision lecture slides.

17

Types of Variables and Univariate Analysis

The following information and Table 1 relate to questions 17 to 20 inclusive.

In a case control study, a team of investigators sought to compare the demographic and cardiovascular risk profile of patients with and without coronary artery disease (CAD). Univariate comparisons are presented in Table 1.

Table 1 - Univariate comparison of characteristics in patients with and without coronary artery disease (CAD):



What type of variable is ‘age’?


  Discrete

  Continuous

  Nominal

  Ordinal

  None of the above

18

What type of variable is ‘sex’?


  Discrete

  Continuous

  Nominal

  Ordinal

  None of the above

19

What type of variable is ‘race’?


  Discrete

  Continuous

  Nominal

  Ordinal

  None of the above

20

With regards Table 1, which of the following statements is/are correct?


  There were 86 patients in the CAD group and 870 patients in the No-CAD group.

  The t-test was most likely used to compare the mean age of patients in each group.

  The mean difference in age between the two groups was 9.4 years, and this difference was statistically significant at the 0.05 level.

  The chi-square test was most likely used to compare the proportion of female patients in each group.

  All of the above.