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Measurements of disease
Amit Gangwar
MVSc. scholar
Tool of measurements
● RATIO
● RATE
● PROPORTION
RATIO
● The value obtained by dividing one
quantity by other - X/Y.
● relations between two quantities.
● Numerator is not a part of denominator.
Ex. - Male : Female ratio
WBC : RBC ratio
RATE
● expresses a change in one quantity (the numerator) with respect to
another quantity (the denominator).
● ‘Time’ is included in the denominator.
● Ex. - Velocity (e.g., 10 m per second) is a rate.
Proportion
● Relation between two quantities.
● Numerator always parts of denominator.
Ex. - prevalence, case fatality are proportions.
Measurements of morbidity
Incidence
● Occurrence of new cases
Prevalence
● Existence of all new & old cases.
Prevalence
number of individuals having a disease at a particular point in time
P = ——————————————————————————————
number of individuals in the population at risk at that point in time
Prevalence
Point prevalence
● the amount of disease in a population at a
particular point in time.
Period prevalence
● number of cases that are known to have
occurred during a specified period of time.
● Ex. - a year (annual prevalence).
INCIDENCE
Two components:
1 - the number of new cases;
2 - the period of time over which the new cases occur.
Cumulative incidence Incidence Rate
● Proportion
number of individuals that become diseased during a particular period
———————————————————————————————
number of healthy individuals in population at beginning of that period
● Rate
number of new cases of disease that occur in a population during a
particular period of time
—————————————————————————————————
the sum, over all individuals, of the length of time at risk of developing
disease
The relationship between prevalence and
incidence rate
● P = I x D
● This means that a change in prevalence can be due to:
1 - a change in incidence rate.
2 - a change in the average duration of the disease.
3 - a change in both incidence rate and duration.
Attack rate Secondary attack rate
● describe the proportion of animals that develop
the disease, when period of risk is brief.
● Rate at which disease is spreading.
● Expressed in %.
● proportion of cases of a transmissible disease
that develop when contact with the primary
case.
Mortality
● When the relevant outcome is death.
● Rather than new case of a specific
disease.
Cumulative mortality
● estimated same as cumulative incidence.
● number of individuals that die during a particular period
number of individuals in the population at the beginning of the period
Mortality rate
● Calculated same as incidence rate.
number of deaths due to a disease that occur in a population particular
period of time
M=
the sum over all individuals, of the length of time at risk of dying
Death rate
● The death rate is the total mortality rate for all diseases
● rather than one specific disease, in a population.
Case fatality rate
● The tendency for a condition to cause the death of affected animals in a
specified time is the case fatality.
● proportion of diseased animals that die.
number of deaths
number of diseased animals
Measures
CRUDE
● Crude prevalence, incidence and mortality
values are an expression of the amount of
disease and deaths in a population as a whole.
● no account of the structure of the population
affected.
SPECIFIC
● describe disease occurrence in specific
categories of the population.
● related to host such as age, sex, breed and
method of husbandry, and, in man, occupation
and socio-economic group.
● convey more information than crude measures
on the pattern of disease.
● can indicate categories of animal that are
particularly at risk of disease, and can provide
evidence on its cause.
Measures
Adjusted (standardised)
● Remove confounding of different age structures.
● Adjusting the crude values.
● reflect the values that would be expected if the potentially confounding
characteristics were similarly distributed in the two study populations.
Mapping
● displaying the geographical (spatial) distribution of disease by
drawing maps (cartography).
● recording of areas where diseases exist but also in investigating
the mode and direction of transmission of infectious diseases.
POINT/DOT MAP
● illustrate outbreaks of disease in
discrete locations, by circles, squares,
dots or other symbols.
● Qualitative.
● Direction of spread of disease.
DISTRIBUTION
MAP
● show the area over which disease
occurs.
PROPORTIONAL
CIRCLE MAP
● Morbidity and mortality can be
depicted using circles whose area is
proportional to the amount of disease
or deaths.
● Shading is used to give the
impression of spheres on a two-
dimensional map.
CHOROPLETHIC
MAP
● Greek: choros = ‘an area’, ‘a region’;
plethos = ‘a throng’, ‘a crowd’, ‘the
population.
● display quantitative information as
discrete shaded units of areas.
● graded in intensity to represent the
variability of the mapped data.
ISO PLETHIC
MAP
● True boundaries between
different values is depicted by
joining all points of equal value
by a line.
Why should we
measure it?
● Describe the extent and nature of
disease n community - assist in
establishing priorities.
● Provide essential data for research.
● Starting point for etiological disease,
help in PREVENTION.
● Monitoring and evaluation of disease
control activities.
Thanks!
Reference:
Veterinary epidemiology, 3rd edition
(Michael thrusfield)

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Measurements of disease.pptx by Dr. Amit gangwar

  • 1. Measurements of disease Amit Gangwar MVSc. scholar
  • 2. Tool of measurements ● RATIO ● RATE ● PROPORTION
  • 3. RATIO ● The value obtained by dividing one quantity by other - X/Y. ● relations between two quantities. ● Numerator is not a part of denominator. Ex. - Male : Female ratio WBC : RBC ratio
  • 4. RATE ● expresses a change in one quantity (the numerator) with respect to another quantity (the denominator). ● ‘Time’ is included in the denominator. ● Ex. - Velocity (e.g., 10 m per second) is a rate.
  • 5. Proportion ● Relation between two quantities. ● Numerator always parts of denominator. Ex. - prevalence, case fatality are proportions.
  • 6. Measurements of morbidity Incidence ● Occurrence of new cases Prevalence ● Existence of all new & old cases.
  • 7.
  • 8. Prevalence number of individuals having a disease at a particular point in time P = —————————————————————————————— number of individuals in the population at risk at that point in time
  • 9. Prevalence Point prevalence ● the amount of disease in a population at a particular point in time. Period prevalence ● number of cases that are known to have occurred during a specified period of time. ● Ex. - a year (annual prevalence).
  • 10. INCIDENCE Two components: 1 - the number of new cases; 2 - the period of time over which the new cases occur.
  • 11. Cumulative incidence Incidence Rate ● Proportion number of individuals that become diseased during a particular period ——————————————————————————————— number of healthy individuals in population at beginning of that period ● Rate number of new cases of disease that occur in a population during a particular period of time ————————————————————————————————— the sum, over all individuals, of the length of time at risk of developing disease
  • 12. The relationship between prevalence and incidence rate ● P = I x D ● This means that a change in prevalence can be due to: 1 - a change in incidence rate. 2 - a change in the average duration of the disease. 3 - a change in both incidence rate and duration.
  • 13. Attack rate Secondary attack rate ● describe the proportion of animals that develop the disease, when period of risk is brief. ● Rate at which disease is spreading. ● Expressed in %. ● proportion of cases of a transmissible disease that develop when contact with the primary case.
  • 14. Mortality ● When the relevant outcome is death. ● Rather than new case of a specific disease.
  • 15. Cumulative mortality ● estimated same as cumulative incidence. ● number of individuals that die during a particular period number of individuals in the population at the beginning of the period
  • 16. Mortality rate ● Calculated same as incidence rate. number of deaths due to a disease that occur in a population particular period of time M= the sum over all individuals, of the length of time at risk of dying
  • 17. Death rate ● The death rate is the total mortality rate for all diseases ● rather than one specific disease, in a population.
  • 18. Case fatality rate ● The tendency for a condition to cause the death of affected animals in a specified time is the case fatality. ● proportion of diseased animals that die. number of deaths number of diseased animals
  • 19. Measures CRUDE ● Crude prevalence, incidence and mortality values are an expression of the amount of disease and deaths in a population as a whole. ● no account of the structure of the population affected. SPECIFIC ● describe disease occurrence in specific categories of the population. ● related to host such as age, sex, breed and method of husbandry, and, in man, occupation and socio-economic group. ● convey more information than crude measures on the pattern of disease. ● can indicate categories of animal that are particularly at risk of disease, and can provide evidence on its cause.
  • 20. Measures Adjusted (standardised) ● Remove confounding of different age structures. ● Adjusting the crude values. ● reflect the values that would be expected if the potentially confounding characteristics were similarly distributed in the two study populations.
  • 21. Mapping ● displaying the geographical (spatial) distribution of disease by drawing maps (cartography). ● recording of areas where diseases exist but also in investigating the mode and direction of transmission of infectious diseases.
  • 22. POINT/DOT MAP ● illustrate outbreaks of disease in discrete locations, by circles, squares, dots or other symbols. ● Qualitative. ● Direction of spread of disease.
  • 23. DISTRIBUTION MAP ● show the area over which disease occurs.
  • 24. PROPORTIONAL CIRCLE MAP ● Morbidity and mortality can be depicted using circles whose area is proportional to the amount of disease or deaths. ● Shading is used to give the impression of spheres on a two- dimensional map.
  • 25. CHOROPLETHIC MAP ● Greek: choros = ‘an area’, ‘a region’; plethos = ‘a throng’, ‘a crowd’, ‘the population. ● display quantitative information as discrete shaded units of areas. ● graded in intensity to represent the variability of the mapped data.
  • 26. ISO PLETHIC MAP ● True boundaries between different values is depicted by joining all points of equal value by a line.
  • 27. Why should we measure it? ● Describe the extent and nature of disease n community - assist in establishing priorities. ● Provide essential data for research. ● Starting point for etiological disease, help in PREVENTION. ● Monitoring and evaluation of disease control activities.
  • 28. Thanks! Reference: Veterinary epidemiology, 3rd edition (Michael thrusfield)