Epidemiology in Public Health

MSc Public Health
Epidemiology in Public Health
(PUB002-6/STI009-6)
Revision Session
Session – 11
22-23, SEM1
Epidemiology Exam
• Assignment – Exam
• 100% of the marks
• ALL examinations will be taken online through
WISEFLOW
https://breo.beds.ac.uk/bbcswebdav/pid-6385772-dtcontent-rid-32879053_1/xid-32879053_1
• Exam date – students are required to check the exam
data/time at the UoB exam office.
https://breo.beds.ac.uk/ultra/organizations/_52351_1/cl/outl
ine > December 2022 SEM1 Examination Timetable
https://breo.beds.ac.uk/ultra/organizations/_52351_1/cl/outline
2
• Timetable and updated guidance on BREO via the
Exams information for Students organisation
Exam date:
Thursday 15 December 2022, 9.00am
(online, remote)
Expectations (from Unit Handbook)
Should be able to demonstrate knowledge of:
• the nature and uses of epidemiology
• the epidemiological approach to defining and measuring the occurrence of
health related states in populations
• the strengths and limitations of epidemiological study designs
• the epidemiological approach to causation
• the contribution of epidemiology to the prevention of disease, the promotion of
health and the development of health policy
• the contribution of epidemiology to good public health/healthcare practice
and
• the role of epidemiology in evaluating the effectiveness and efficiency of
health care.
Epidemiology is the study of the distribution and determinants of health-related states or events (including diseases and other health conditions) in specified populations, and the application of this study to the control of health problems.

The epidemiological approach involves the use of statistical and analytical methods to describe and measure the occurrence of health-related states in populations. This includes the study of patterns of disease, the identification of risk factors for disease, and the evaluation of the effectiveness of interventions designed to prevent or treat disease.

One of the strengths of epidemiological study designs is that they can be used to study large populations over time, allowing for the identification of trends and patterns in disease occurrence. Epidemiological studies can also be used to identify risk factors for disease, which can help to inform the development of interventions to prevent or mitigate the impact of those diseases.

However, epidemiological studies have limitations as well. For example, they may not be able to establish a causal relationship between a risk factor and a disease, as other factors may also be contributing to the observed relationship. Additionally, epidemiological studies may be subject to bias and confounding, which can affect the validity of the results.

Despite these limitations, epidemiology plays a critical role in the prevention of disease, the promotion of health, and the development of health policy. By identifying risk factors for disease and evaluating the effectiveness of interventions, epidemiology can help to inform the development of strategies to prevent or control the impact of diseases on populations. Epidemiology is also important for evaluating the effectiveness and efficiency of healthcare practices, helping to ensure that resources are used effectively to improve the health of populations.
In addition, the student will be expected to have gained a variety of skills,
including an ability to:
• outline appropriate study designs to answer specific questions concerning
disease causation, natural history, prognosis, prevention, and the evaluation
of therapy and other interventions to prevent and control disease.
Epidemiology Exam
• Epidemiology
 Short answer questions
 Multiple choice questions
 Calculation and interpretation
 Essay (type) question
3
General advice
• This is a CLOSED-BOOK exam. You are expected to work on
your own to complete this exam, not using other people or any
materials for your assistance. Marks will NOT be given for
repetition of standard material from notes or books or websites.
• Read the exam paper carefully – making sure that you
understand instructions. All questions do not necessarily carry
equal marks.
• Give precise, specific answers that demonstrate the depth of
your knowledge of the area being examined by the question,
rather than vague and general comments.
• General advice – try to be concise and accurate.
Possible short questions
– Prevalence
– Incidence
– Advantages/disadvantages of
routine data
– Population at risk
– Crude mortality rate
– Age standardisation rates
– Prevention and types
– Confounding factor
– Cross-sectional study
– Ecological fallacy
– Sensitivity
– Specificity
– Selection of control group in a
case-control study
– Advantages/disadvantages of
case-control study
– Advantages/disadvantages of
cohort study
– Selection bias
– Observer bias
– Recall bias
– Attrition bias
– Matching
– Population screening
– Criteria of a disease suitable
for screening
– Criteria for the assessment of
causality
– Confounding
– Controlling for confounding
effect in the design and in the
analysis
Risk, Rate and Odds
Risk
1. Defined population
2. Defined period of time
3. Number of new cases
Rate
1. Number of new cases
2. The person time (cumulative Person-time)
Odds
1. Number of events happening
2. Number of events not happening
4
Risk and odds
Risk:
• 24 people drank coffee
• 6 developed a headache
• Risk of headache?
Odds:
• 24 people drank coffee
• 6 developed a headache
• Odds of headache?
Risk
• 24 people drank coffee
• 6 developed a headache
Risk = no. of participants WITH event of interest
total no. of participants
= 6 headaches/24 people
= 6/24 = ¼= 0.25 = 25%
Interpretation: The risk of having a headache was one in four, or 25%
Odds
• 24 people drank coffee
• 6 developed a headache
Odds = no. of participants WITH event of interest
no. of participants WITHOUT event of interest
= 6 headaches/18 without headaches
= 6/18 = 1/3 = 0.33 = 33% (1:3)
Interpretation: The odds of having a headache were one in third of the odds of not
having a headache OR one person had a headache for every three that didn’t OR the
odds of having a headache were 3 to 1 against
Odds-Ratio and Risk-Ratio
• Odds-Ratio is the ratio of odds
 An odds is the probability of failure over probability of
success.
• Risk-ratio is the ratio of risks
 A risk is simply the probability of failure
• An odds-ratio or risk-ratio of 1 indicates no
difference between the groups (e.g. disease/riskoutcome)
• Odds-ratio more difficult to interpret than risk-ratios
• Risk ratios are more sensitive to base event rate;
odds-ratios are not
(Lipsey and Wilson, 2000)
5
Comparing two groups
Task: Calculate the effect measures and interpret
them:
 Risk-ratio (RR) [relative risk]
 Odds ratio (OR), if it’s a case-control study.
 Risk difference (RD) [absolute risk reduction]

Type Headache No headache Total
Caffeine 17 51 68
Decaf 9 55 64
Total 26 106 132
Comparing two groups
Calculate the effect measures
Risk-ratio (RR) [relative risk]
= (risk of event with intervention)/(risk of event with
control)
= (17/68)/(9/64) = 0.25/0.14 = 1.79
[Where RR =1, there is no difference between the groups]
• Interpretation: the risk of having a headache with treatment
was 179% of the risk in the control group OR intervention
increased the risk of headache by 79%

Headache No headache Total
Caffeine (Inter.) 17 51 68
Decaf (contrl) 9 55 64
Total 26 106 132
Comparing two groups
• Odds ratio (OR)
• OR = (17×55)/(9×51) = 935/459 = 2.037 = 2.04
[Where OR =1, there is no difference between the groups]
• Interpretation: Intervention doubled the odds of headache OR
intervention increased the odds to 204% of the odds in the
control group OR intervention increased the odds of headache
by 104%

Headache No
headache Total
Caffeine 17 51 68
Decaf 9 55 64
Total 26 106 132
6
Comparing two groups
Calculate the effect measures
• Risk difference (RD) [absolute risk reduction]
• RD= Risk-ratio (RR) [relative risk]
• = (17/68)-(9/64) = 0.25-0.14 = 0.11
[Where RD =0, there is no difference between the groups]
• Interpretation: intervention increased the risk of headache by
11%

Headache No
headache Total
Caffeine 17 51 68
Decaf 9 55 64
Total 26 106 132
Prevention
Choice of study designs
• Investigation of rare disease………..
• Investigation of rare cause………..
• Measure of incidence……..
• Measure of prevalence……..
7
Choice of study designs
Measure of prevalence…….. Cross-sectional
Useful 2 x 2 table for calculating rates, odds,
prevalence, relative risks, odds ratios
8
Final tips:
1. Revise – weekly teaching notes/slides
2. Also look at these texts:
Basic epidemiology, Bonita, Beaglehole and Kjellström (WHO)
https://apps.who.int/iris/bitstream/handle/10665/43541/9241547073_eng.pdf;jsessionid=986D1CBB27D8AEFDCD290F9BDC105
4A7?sequence=1
Gordis Epidemiology
Epidemiology for the Unintended
https://www.bmj.com/about-bmj/resources-readers/publications/epidemiology-uninitiated

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