Measurements in Epidemiology, Schemes and Mind Maps of Environmental Science

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2. Measurements in

Epidemiology

2. Measurements in

Epidemiology

UNIT IV Fundamentals of Epidemiology

Objectives

• (^) By the end of the session you will be able to understand and explain:
  1. scope of measurements in epidemiology
  2. basic requirements of measurements
  3. Concept of numerator and denominator
  4. Tools of measurement :
     1. Rate
     2. ratio
     3. Proportion
  5. Measures of natality
  6. Measures of mortality
  7. Measures of morbidity
     1. Incidence
     2. prevalence

a) Measurement of mortality ( death) b) Measurement of morbidity- ( having a disease or a symptom of disease) c) Measurement of disability- (a physical or mental condition that limits a person's movements, senses, or activities.) d) Measurement of natality- ( related to birth) e) Measurement of the presence, absence or distribution of the characteristic or attributes of the disease f) Measurement of medical needs, health care facilities, utilization of health services and other health-related events g) Measurement of the presence, absence or distribution of the environmental and other factors suspected of causing the disease. h) Measurement of demographic variables

Basic Requirements Of Measurements

• (^) The basic requirements of measurements are:
  • (^) reliability/Repeatability/precision/reproducibility: consistent results when repeated more than once
  • (^) Validity (accuracy): to what extent the test accurately measures which it purports to measure.
  • (^) Sensitivity – true positive
  • (^) Specificity – true negatives Important

• (^) a. Related to the population: The denominators related to the population comprise the following:
• (^) Mid-year population:
• (^) Population at risk
• (^) Person -time:
• (^) Person -distance:
• (^) Sub -groups of the population:
• (^) b. Related to total events: In some instances, the denominator may be related to total events instead of the total population, as in the case of infant mortality rate and case fatality rate.

Tools of measurement

• (^) The epidemiologist usually expresses disease magnitude as - (^) 1. Rates - (^) 2. Ratios - (^) 3. Proportions

• (^) rates are particularly useful for comparing disease

frequency in different locations, at different times,

or among different groups of persons with

potentially different sized populations; that is, a

rate is a measure of risk.

• (^) It is a statement of the risk of developing a

condition.

• (^) The various categories of rates are:
• (^) (1) Crude rates: These are the actual observed rates such as the birth and death rates. Crude rates are also known as unstandardized rates.
• (^) (2) Specific rates: These are the actual observed rates due to specific causes (e.g., tuberculosis); or occurring in specific groups (e.g., age-sex groups) or during specific time periods (e.g., annual, monthly or weekly rates).
• (^) (3) Standardized rates: These are obtained by direct or indirect method of standardization or adjustment, e.g., age and sex standardized rates

 (^) Ratios are common descriptive measures, used in all fields  (^) In epidemiology, ratios are used as both descriptive measures and as analytic tools. e.g. -male-to-female ratio of participants in a study can be calculated for occurrence of illness, injury, or death between two groups. -risk ratio (relative risk), and odds ratio, etc.

Example : A city of 4,000,000 persons has 500 clinics. Calculate the ratio of clinics per person. Solution - 500 / 4,000,000 x 10n = 0.000125 clinics per person 500 x 10, 4,000,000. Then the ratio becomes: 1.25 clinics per 10,000 persons express this ratio as 1 clinic for every 8,000 persons.

3. PROPORTION:

• (^) A proportion is a ratio which indicates the relation in magnitude of a part of the whole.
• (^) The numerator is always included in the denominator. A proportion is usually expressed as a percentage.

• It is a type of ratio in which the numerator is

always a subset of the denominator.

Important

• (^) Example: [The number of children admitted with pneumonia at a certain time / The total number of children admitted in the hospital at the same time] x 100

 (^) In epidemiology, proportions are used most often as descriptive measures eg - the proportion of children in a village vaccinated against measles,  (^) also used to describe Attributable risk the amount of disease that Measures of Risk can be attributed to a particular exposure.

 (^) Proportions can easily be converted to ratios. eg - out of the 341 pt. who attended clinic ,the number of women were (179), & the proportion of clinic attendees who are women is ………., & ratio of women to men could be calculated ……………?? Proportion = 179/341=52%, Ratio = 179 / (341 – 179) x 1 = 179 / 162 = 1.1 to 1 F:M ratio