Epidemiological studies concept in ophthalmology. This was a presentation done on zoom for the Magrabi ICO Cameroon Eye Institute, Cameroon and Center for Sight Africa, Nigeria. It deals with conceiving a research question, deciding the target population, measures of burden of disease such as prevalence and incidence, measures of associations between exposure and disease- relative risks, confidence intervals, threats to making inferences such as confounders, Bias, and chance, Bradford-Hill guideline, cross sectional study, cohort study, Case-control study, Randomised controlled trials and it concludes with types and use of epidemiological studies.

1. Arnold Mofor, MD
PGY2 WACS Ophthalmology Residency Programme,
Magrabi ICO Cameroon Eye Institute
1

2. Quote by Dr. Arthur C. Guyton
“People who are really successful in the research world
are self-taught”
2

3. Epidemiology and Truth Finding
3

4. Significance of review
 Informs on type and
use of data collected.
 Acquiring skills for critical
reading an article.
 Gaining proficiency in
communicating research
findings.
 We reap what we sow in designing a study
to find disease burden, cause, and interventions
4

5. Epidemiological Research
Questions
 Questions must be
FINER (feasible, interesting,
novel, ethical, relevant).
 How common is a disease?
 Who is most affected?
 What is the underlying cause?
 What can be done to prevent of treat it?
5

6. Target Population
 Defined by common clinical, demographic,
geographical and temporal characteristics.
 A manageable sample for study is taken from the
target population forming its subset.
6

7. Measures of Burden of Disease-
Prevalence
 Calculates the estimated number of persons with
disease in a population at a point in time.
 Study in Bangladesh with a sample = 11624, blind
persons (VA <3/60) = 162, Prevalence of blindness =
162/11624 (1.39%).
7

8. Measures of burden of disease-
Incidence
 Relates to new cases.
 Two measures exist; Cumulative Incidence and
Incidence rates.
 Cumulative incidence = number of new cases
occurring in a population which is disease free at
baseline.
8

9. Measures of burden of disease-
Incidence
 Incidence Rates:
 Uses person time
at risk rather than
population at risk.
9

10. Measures of Associations
between Exposure and Disease
 Answers questions related
to aetiology and efficacy.
 Compares prevalence or
incidence in different groups
defined by exposure of interest.
 The ratio of the prevalence or
incidence in one group compared
to the other is computed.
10

11. Measures of Associations
between Exposure and Disease
 Relative Risk (RR):
 Refers to cumulative incidence
ratio or incidence risk ratio.
 Odds ratio compares odds of
exposure between those with
and those without disease.
 RR = 1.0 (no association exist).
 RR = > 1.0 (exposure may cause disease).
 RR = < 1.0 (exposure may prevent disease).
11

12. Confidence Intervals
 Important because of variability due to chance.
 It expresses the degree of confidence we have in our
estimate gotten from a sample.
 It provides the upper and lower range of our estimate.
 95% confidence interval for the sample estimate means
the true value for the population lies in this range 95% of
the time.
12

13. Threats to making inferences
 Chance:
 Inevitable in any study.
 Managed by having a sufficient enough sample or
power.
13

14. Threats to making inferences
 Confounders:
 Alternative explanations to the association.
 Controlled in the design by ensuring sample has
similar characteristics or randomisation.
 Controlled in the analysis through multivariate
regression or stratified analysis.
14

15. Threats to making inferences
 Bias: Selection or Information bias’.
 Selection bias = systematic difference
between study groups.
 Information bias is caused
by inaccuracy in measurement
of exposure or disease.
 Minimised by good study
design and paying attention to
the quality of data collection.
15

16. Case for Exposure-Disease
Association
 Bradford-Hill guideline:
 Temporality
 Strength of evidence
 Coherence
 Experimental evidence
 Plausibility
 Dose-response relation
 Specificity
 Analogy
16

17. Study Conception
17

18. Cross Sectional Survey
 Measures the prevalence of disease.
 We interview or examine
the study sample to find
out if they have the disease
or exposure of interest.
 Associations are difficult to
establish with this design
due to difficulty to test
temporarility.
18

19. Cohort Study
 Allow us to measure predictors of disease incidence.
 Prone to loss to follow-up, competing risk, expensive
and time consuming.
 Weaknesses reduced by using an incidence rate
analysis (person time analyses).
19

20. Case-Control Studies
 Used to also establish disease aetiology.
 Cases have the disease, Controls do not.
 Cannot estimate the burden of disease.
 More than one control to case
increases the statistical power.
 Quick and cheap, investigates
rare diseases.
 Prone to recall
and selection bias.
20

21. Randomised Controlled Trials
 PICO format guides RCT
research questions
 Assess benefit of a new
drug or intervention.
 Evaluate the impact of a
preventive measure.
 Randomisation makes the intervention
and control groups very similar.
 Weaknesses include loss to follow-up,
treatment refusal, administering other drugs.
 Intention-to-treat analysis and blinding handles
weaknesses.
21

22. Take Home Message
22

23. THANK YOU FOR YOUR ATTENTION
23

24. References
Kuper H, Gilbert C. Epidemiology for Ophthalmologists:
an introduction to concepts, study designs, and
interpreting findings. Br J Ophthalmology 2005;89:378-
384.
24