CM: Epidemiology and biostatistics

Question 1

Definition of epidemiology

Answer 1

study the occurence and distribution of disease-related events, states, and processes in specified population, including the study of determinants, and apply the knowledge to control relevant health problems

Question 2

Roles of epidemiology (2)

Answer 2

1. describe disease pattern (descriptive)
2. identify disease determinants (analytical)

Question 3

Definition of prevalence

Answer 3

number of existing cases / population of interest at a designated point of time

Question 4

Advantages of using prevalence (3)

Answer 4

• time- and resource-saving
• quickly assessing current health needs, and useful to anticipate future needs
• generate ideas for future studies / formulate research hypothesis

Question 5

Limitations of using prevalence (3)

Answer 5

• good for chronic diseases but not for disease in a short duration
• cannot tell the number of new cases
• some cases are not present / detected at the time of study (e.g. early death, cured, sub-clinical)

Question 6

Reasons for false changes in disease frequency (4)

Answer 6

• better diagnostic method
• change in diagnostic criteria
• increase in screening coverage
• change in law (compulsory reporting)

Question 7

Descriptive studies vs Analytical studies

Answer 7

D: describe disease pattern and distribution, for formulating research hypothesis and better allocating public health resources
- e.g. Case study, Correlational study, Cross-sectional study

A: identify disease determinants, for evaluating the casual relationship between exposure and outcomes
- e.g. Cohort study, Case-control study, RCT

Question 8

Advantages of case report (3)

Answer 8

• alert public health workers for new disease / new therapy
• risk communication
• formulate research hypothesis

Question 9

Definition of case report

Answer 9

detailed clinical report by doctors of a single patient with unusual clinical features

Question 10

Limitations of case report (3)

Answer 10

• no control
• small sample size
• limited generalizability and interpretability

Question 11

Steps of conducting a cross-sectional study

Answer 11

1. select a study sample, as a representative of the population of interest
2. simultaneously determine both the exposure and disease status, with no follow up
3. construct a contingency table to calculate the prevalence in exposure & non-exposure groups, and thus the relative risk

Question 12

Pros of cross-sectional studies (3)

Answer 12

(same as the advantages of using prevalence)

• time- and resource-saving
• quickly assessing current health needs, and useful to anticipate future needs
• generate ideas for future studies / formulate research hypothesis

Question 13

Cons of cross-sectional studies (3)

Answer 13

• use of prevalence: detectable cases may not be representative of all cases; not cost-effective for diseases with short duration
• lack of follow up: may misclassify exposure or outcome groups
• no temporal sequence –> cannot establish casuality between exposure & outcome

Question 14

What is cohort?

Answer 14

designated group of individuals who are followed up over a period of time

Question 15

What is birth cohort?

Answer 15

a group of people who born in the same year, being followed up over a period of time

Question 16

What is cohort study?

Answer 16

a longitudinal study, in which people who are (a) initially disease-free but (b) at risk of the disease are followed up over time, to (1) measure the incidence of the population and (2) test the casual relationship between exposures and outcomes

Question 17

Cumulative incidence v.s. Incidence rate (in cohort study)

Answer 17

CI = # of new cases / # of individuals at risk (used in closed cohort)

IR = # of new cases / sum of person-years of individuals at risk (used in open cohort)

Question 18

4 steps of conducting cohort study

Answer 18

1. Select study samples (initially disease-free, and at risk)
2. Determine exposure status
3. Follow up and measure incidence of the disease
4. Measure association

Question 19

Advantages of prospective vs retrospective cohort studies

Answer 19

P: (1) Better control of quantity and quality of the data; (2) less potential for bias

R: (1) Less expensive; (2) Less time-consuming

Question 20

How to obtain high FU rate in cohort studies?

Answer 20

1. Remove barriers to FU
2. Give incentives
3. Tracking software

Question 21

Pros of cohort studies (5)

Answer 21

• Able to measure the incidence
• Able to study multiple exposures
• Able to test on harmful exposures
• Clear temporal sequence
• Less potential to recall bias

Question 22

Cons of cohort studies (5)

Answer 22

• Large sample size
• Expensive
• Long FU time
• Loss to FU
• Inefficient for rare outcomes

Question 23

When to use case-control studies? (3)

Answer 23

• rare disease
• long latent period
• little knowledge about the causes so multiple risk factors are of interest

Question 24

4 Steps of conducting a case-control study

Answer 24

1. Case selection
2. Control selection
3. Measure exposures
4. Measure association

Question 25

3 Criteria of confounders

Answer 25

1. Independent risk or protective factor of the disease
2. Associated with the main exposure
3. Not on the casual pathway between exposure & outcome

Question 26

Controls of confounders (3+2)

Answer 26

• restriction, randomization, matching
• stratification, multi-variable statistical analysis

Question 27

Pros of case-control study (5)

Answer 27

• Cases easily available
• Efficient for rare diseases
• Quick, relatively inexpensive
• Able to measure multiple exposures
• Small sample size

Question 28

Cons of case-control study (5)

Answer 28

• Time sequence may not be clear
• Cases may be selective survivors
• Able to measure one outcome only
• Recall bias
• Difficult to select appropriate controls

Question 29

What is external validity?

Answer 29

Generalizability of a study result to be applied to the population