Week 1

Question 1

What is epidemiology?

Answer 1

The study of the distribution and determinants of disease in human populations

Question 2

Define prevalence:

Answer 2

Existing cases of an outcome of interest (e.g. disease), in a defined population at one point in time.

Question 3

How is prevalence expressed?

Answer 3

As a proportion or percentage

Question 4

Define point prevalence:

Answer 4

Prevalence literally at one point in time

Question 5

Define period prevalence:

Answer 5

Prevalene measured over a (short) period of time (e.g. over the last year)

Question 6

Define incidence:

Answer 6

Number of new cases of an outcome of interest (e.g. disease) arising from a defined population, during a time interval

Question 7

How is incidence expressed?

Answer 7

As a rate (denominator has a time component)

Question 8

Define risk:

Answer 8

Probability of disease occurring in a disease-free population during a specified time period

Question 9

How is risk calculated?

Answer 9

Risk= n/P
n= new cases in a defined period
P= population at risk

Question 10

What is a limitation of using risk as a measure?

Answer 10

Assumes that all of the population were followed up/at risk for the full time period, and ignores that follow-up periods for different people are often different.

Question 11

Define rate:

Answer 11

Probability of disease occuring in a disease-free population during the sum of individual time periods.

Question 12

How is rate calculated?

Answer 12

Rate= n/ total person-time of follow up

Question 13

What is a benefit of calculating rate instead of risk?

Answer 13

Denominator makes explicit the time that the ‘population at risk’ spent being ‘at risk’.

Question 14

What does person-time involve counting?

Answer 14

The exact time periods that participants were actually followed up for, added up together.

Question 15

Explain cumulative incidence:

Answer 15

• Measured by risk
• Assumes that population-at-risk has been followed up for the entire specified period of time
• Used in closed cohort studies

Question 16

Explain incidence density:

Answer 16

• Measured by rate
• Denominator expressed as person-time
• Used in open cohort study designs

Question 17

What does crude risk/rate apply to?

Answer 17

The whole population (e.g. rate of heart disease in Australia)

Question 18

What is category specific risk/rate?

Answer 18

Applies to a specific sub-population (e.g. rate of heart disease in Australian women aged 35-45 years)

Question 19

What is one limitation of calculating crude rate?

Answer 19

Does not take into account differences in sub-population, e.g. age, which is a significant confounder for illness and death.

Question 20

Define standardisation:

Answer 20

A process which adjusts for confounding factors (usually age) when comparing rates.

Question 21

What does standardisation allow for?

Answer 21

The comparison of two populations, taking into account their different age distributions

Question 22

What are the two types of standardisation?

Answer 22

Direct and indirect

Question 23

What are the steps in direct standardisation?

Answer 23

1. Stratify population by age
2. Calculate age-specific rates to a reference population by age
3. Apply age-specific rates to a reference population stratified by age
4. Recalculate age-standardised rate

Question 24

Explain the benefits and limitations of direct standardisation:

Answer 24

• Provides summary rates (all ages) that remove the unwanted effects of differences in the distribution of confounders in the population
• However the adjusted rates are not real
• Allows a fairer comparison after ironing out some of the other differences that may be exaggerating or masking differences in the population
• May involve more than two groups or populations

Question 25

When is in-direct standardisation commonly used?

Answer 25

When age-specific rates are unavailable

Question 26

What are the steps in indirect standardisation?

Answer 26

1. Stratify study population by age
2. Calculate expected number of deaths using standard rates applied to your population
3. Calculate SMR (observed/expected)
4. Recalculate age-standardised rate (crude rate of ref population by SMR)

Question 27

Define and explain CMR:

Answer 27

Crude mortality rate:
CMR= (# deaths in population per time period/total population in time period) x 100,000

CMR depends on mortality rate in each age group, as well as proportion of people in age group in the population

Question 28

Define and explain SMR:

Answer 28

Standardised mortality ratio:
SMR= (observed number of deaths/expected number of deaths) x 100%
Ratio tells us how the death rate in the study compares to the standard population
Allows for comparison of ‘health’ between populations

Question 29

Define morbidity rate:

Answer 29

The incidence rate of non-fatal cases in the total population at risk during a specified time period

Question 30

Define mortality rate:

Answer 30

The incidence rate of fatal cases in the total population at risk in a specified time period.
Mortality rate= number of deaths/total population

Question 31

Define case fatality rate:

Answer 31

A measure of the severity of the disease.

Calculated by dividing number of deaths by all those who have the disease.

Question 32

Define crude rate:

Answer 32

Rates that apply to an entire population without reference to any specific characteristic about the population.

Question 33

Explain direct standardisation:

Answer 33

• Used to remove the biasing effect of age due to differing age structure of different populations
• Involves calculating the overall incidence or mortality you would have expected to find in a standard population if it had the same age-specific rates as your population
• Requires age-specific rates
• More commonly used than indirect

Question 34

Explain indirect standardisation:

Answer 34

• Used if age specific rates are not available in the population requiring adjustment
• Uses rates derived from the standard population and applies them to the known age groups in the population to be standardised