QUESTION 1:
In 2013, a group of 3602 men and women aged 15-30 years were recruited from the North West province of South Africa and assessed for human immunodeficiency virus (HIV). 160 participants were found to have HIV when tested at baseline. Participants who did not test positive for HIV in 2013 were followed-up and tested again after 3 years of follow-up (only those who previously had tested negative for HIV were tested again at follow-up). A further 76 cases of HIV were identified during this 3 year period (either diagnosed during the interim or diagnosed during the study assessment).
Data from this study are presented in the table below, stratified by sex.
No. of participantsrecruited Prevalent cases at baseline Prevalent cases remaining at follow-up* Person years of follow-up Loss to follow-up in those HIV negative at baseline Deaths in those HIV negative at baseline Incident HIV cases at follow-up
Males 1,693 45 37 2,530 143 4 30
Females 1,909 115 107 2,632 117 6 46
Total 3,602 160 144 5,162 260 10 76
*It is assumed that once participants contract HIV they remain HIV positive, and that over the follow-up period some of these cases detected at baseline were lost to follow-up.
(a) What is the study design used in this study? [1 mark]
(b) What was the prevalence of HIV in (i) males and (ii) females aged 15-30 in 2013? [3 marks]
(c) What was the prevalence of HIV in (i) males and (ii) females after 3 years of follow-up? [3 marks]
(d) What was the cumulative incidence of HIV among (i) males and (ii) females over the 3 year study period? [4 marks]
(e) What was the incidence rate of HIV among (i) males and (ii) females for the 3 years? [4 marks]
(f) Is cumulative incidence or incidence rate a better reflection of the incidence of HIV in this study population? Please justify your answer. [2 marks]
(g) Describe the findings on HIV from this study. How would you interpret these findings? (Limit of half a page maximum. No more than half a page of text will be marked) [2 marks]
(h) Since the completion of this study, the drug Truvada, also known as pre-exposure prophylaxis (PrEP), has been introduced in other countries to reduce the acquisition of HIV in those who regularly take the drug. You are asked to brief the South African health minister on the likely effects on the (i) short-term and long-term prevalence and (ii) incidence of HIV of a government-funded pilot program that would make Truvada (PrEP) freely available to those aged 15-30 years in South Africa. What do you tell her? (Limit of half a page. Please note: no more than half a page of text will be marked) [3 marks]
QUESTION 2:
Question 2 relates to the following table of state-level data male suicide, stratified by quintile of socio-economic status (SES). (Only the lowest and highest SES groups are shown). Note: 5 years of death data are presented. Population data comes from a single year (2014). It is conventional to present mortality rates as annual average rates.
Lowest SES quintile, Suicides Lowest SES quintile, Population Highest SES quintile, Suicides Highest SES quintile, Population Total suicide (All quintiles) Total population (All quintiles)
2012-2016 2014 2012-2016 2014 2012-2016 2014Males
15-24 177 118,059 115 134,231 742 635,760
25-34 220 127,908 157 128,186 995 660,941
35-44 185 126,898 122 129,352 824 656,037
45-54 134 105,410 92 118,644 566 549,646
55-64 81 79,946 55 71,722 347 363,093
65-74 69 70,009 42 53,426 270 289,739
75+ 51 37,331 43 34,565 214 164,625
TOTAL 917 665,561 626 670,126 3,958 3,319,841
(a) Calculate the crude suicide mortality rate (per 100,000) for (i) the lowest SES quintile and (ii) the highest SES quintile for those aged =15 years. [4 marks]
(b) Calculate the crude mortality rate ratio (expressed as a %) for suicide in the lowest SES quintile relative to the highest SES quintile [2 marks]
(c) How would you interpret the crude mortality rate ratio you calculated in Question 2b? [2 marks]
(d) How many deaths from suicide would you expect in the highest SES quintile 25-34 year olds if they had the same suicide mortality rate as 25-34 year old lowest SES quintile? [1 mark]
(e) Calculate the direct age-standardised suicide mortality rate (per 100,000) for the lowest SES quintile using the highest SES quintile as the standard [4 marks]
(f) Is age an important factor in understanding differences in suicide between low and high SES groups? Using the age-specific data provided, make a specific comment about the differences between the crude suicide rates and the direct age-standardised suicide rate. [4 marks]
QUESTION 3:
For the following abstracts, please indicate (i) what the study design is and (ii) provide a justification for how you determined the study design
(a) National level data on dietary fish consumption from 15 countries was obtained from the Food and Agricultural organisation. National level osteoarthritis prevalence data was obtained from national health surveillance records from the same countries and age standardized. The correlation between fish consumption and osteoarthritis was calculated to examine whether fish consumption is associated with risk of osteoarthritis. [2 marks]
(b) Queensland Health conducted a telephone survey of randomly selected Queenslanders to examine the prevalence of lifestyle risk factors (e.g. physical activity, diet) and a number of chronic diseases (e.g. heart disease, cancer, arthritis). They also calculated the strength of associations between each lifestyle factors and each chronic disease. [2 marks]
QUESTION 4:
The following table shows data from an epidemiological study:
Exposure status Number of cases of disease Person-years (py) at risk
Present 700 1950
Absent 300 2250
Total 1000 4200
(a) What is the likely study design? [1 mark]
(b) Calculate the relative risk of disease associated with having the exposure [1.5 marks]
(c) Calculate the rate difference (per 100 person years) [1.5 marks]
(d) How would you interpret this rate difference? [2 marks]
(e) How many cases of disease (per 100 person years) in the total study population can be attributed to the exposure [2 marks]
QUESTION 5:
A study was conducted to investigate the effect of recent cannabis use on the risk of being injured in a road traffic accident.
Researchers recruited 488 people (aged 18+) who had been involved in a road traffic accident and presented to an Emergency Department at any of the major hospitals in Brisbane.
At the same time, researchers also recruited 488 people (aged 18+) who had not been injured in a road traffic accident. These participants were also recruited from the same Brisbane hospital Emergency Departments as those in injured in traffic accidents, but were being treated for causes other than road traffic accidents.
A total of 65 people tested positive for cannabis. 42 of these people were involved in road traffic accidents.
(a) What is the study design [1 mark]
(b) In your own words, what are the key advantages of this study design? [1 mark]
(c) Construct a 2x2 table conveying the information above and calculate an appropriate relative measure of the strength of the association between cannabis use and being injured in a road traffic accident [3 marks]
(d) How would you interpret your findings in Question 5c? [1 mark]
(e) Calculate the population attributable fraction of road traffic accidents that is associated with cannabis use [2 marks]
(f) How would you interpret the finding in Q5e? [2 marks]
(g) This study was critiqued by other researchers for not selecting an appropriate comparison group (i.e. those not injured in road traffic accidents). Do you agree with this comment? Please justify your answer by describing characteristics of the comparison group and the study design principles of comparison group selection.
[3 marks]
Sample Solution