Session 2: Rates and vital statistics

Question 1

Define the concept of prevalence.

Answer 1

The number of people who have a particular disease at a specific time.

Question 2

Define the concept of incidence.

Answer 2

The number of new cases reported during a specified period in a defined population.

Question 3

Discuss the use of prevalence.

Answer 3

• Focuses on existing cases, at a specified time
  regardless of the onset time of the disease
• Also known as “point prevalence”, as it refers to the
  number of cases at a point in time
• Period prevalence includes the number of cases that
  existed at any point during a specified period
• Prevalent cases include new cases as well as cases
  that occurred at any time previously, as long as the
  individuals still have the condition at the time of the
  assessment
• Cases who have died or recovered by the time of
  assessment are excluded

Question 4

Discuss the use of incidence.

Answer 4

• Number of new cases can be determined by making
  use of routine surveillance systems, like notifications
• Studies which follow individuals up for a specified
  time can also be conducted to determine the number
  of incident cases

Question 5

Define the concept of rates.

Answer 5

It is calculated by dividing the number of cases by the population at risk and the period of observation

Question 6

Define the concept of ratios.

Answer 6

Quantities such as the infant mortality rate, but terms such as “rates” are entrenched.

Question 7

Define the cumulative incidence.

Answer 7

The proportion of the at-risk population who become diseased in a specified period.

Question 8

Define the incidence rate.

Answer 8

Takes into account the length of disease-free time each individual in the at-risk population remained under observation.

Question 9

Explain what a crude rate is.

Answer 9

If the total number of cases in a population is divided by the size of the population at risk, the crude (overall) rate is calculated.

Question 10

Explain what a specific rate entails.

Answer 10

The epidemiologist determines the rates for subgroups and it enables the researcher to study the variation of the condition of interest between subgroups of a population.

Question 11

Define standardisation of rates.

Answer 11

A way of removing the effect of differences in the composition of various populations and so overcoming the problem of making erroneous comparisons based on crude rates.

It provides new summary rates which take into account the differences between the groups.

Question 12

Define demography and discuss demographic concepts that are important in epidemiology.

Answer 12

Definition: A scientific description of the characteristics of a populations and embraces all aspects of population structure and changes that can be measured numerically.

It primarily involves measurement of the size of the population, composition, distribution and changes in the number of people.

Important demographic concepts in epidemiology:
Fertility, mortality and migration are the main demographic processes which lead to changes in the population size and composition.

Question 13

Define a population pyramid.

Answer 13

A useful summary representation of certain aspects of population composition. It is a graphical display of the percentage composition of a population by age and gender.

Question 14

Name the three demographic processes that determine the changing size of a population.

Answer 14

1. Migration
2. Fertility
3. Mortality (deaths)

Question 15

List five types of demographic processes.

Answer 15

1. Fertility
2. Mortality
3. Life expectancy
4. Migration and urbanisation
5. Demographic transition
   and epidemiological
   transition

Question 16

Define crude birth rate.

Answer 16

The number of live births per 1000 persons in the population.

Question 17

Define the general fertility rate.

Answer 17

The number of births per 1000 women aged 15- 44 years.

Question 18

Define age-specific fertility rate.

Answer 18

If the numbers of the female population and births are known by age of the mother, age-specific fertility can be calculated.

Question 19

Define the total fertility rate.

Answer 19

Adds together the age-specific fertility rates over the reproductive ages.

Question 20

Describe South African two sources of routine health information and their limitations.

Answer 20

1. Routinely collected data:
   - relates to patients seen at health facilities
   - can generate and answer research questions on
   exposures of the natural history of new diseases
   * routine primary care facility data
   * routine hospital data
   * other sources of routine data

Limitations of routinely collected data:
- missing data, incomplete data, data recording problems, differential reporting owing to resource differences, duplicate data, data capture errors, inaccurate denominator data for rates

2. Survey/ research data
   - can improve surveillance by paying great attention to accurate data collection, clear data definitions and by defining and answering specific research questions that cannot be answered by routine data.
   - includes patient interviews, door-to-door interviews, record interviews within facilities
   - resource-intensive
   - can collect data on chronic non-communicable disease, e.g. mental health problems

Limitations of survey/ research collected data:

• selection bias may occur
• information bias

Question 21

Define a health indicator.

Answer 21

A measurable factor that estimates the size of a problem and its determinants and evaluates progress towards a target.

Question 22

Define surveillance.

Answer 22

Process of systematic collection, collation and analysis of data with prompt dissemination to those who need to know for relevant action to be taken.

Question 23

Discuss the functions of surveillance.

Answer 23

• Estimates the magnitude of the problem
• Determines geographic distribution of the illness
• Portrays the natural history of the disease
• Detects epidemics/defines a problem
• Generates hypotheses and stimulates research
• Evaluates control measures
• Monitors changes in infectious agents
• Detects changes in health practices
• Facilitates health planning

Question 24

Provide appropriate examples of surveillance systems.

Answer 24

• Chronic diseases
• Injury surveillance
• Maternal death notification system
• Expanded programme on immunisation surveillance