Measures in Epidemiology

Measures in Epidemiology

# 1. Types of Measures:

• Ratio: A type of fraction where x and y are independent variables.

• Proportion: A type of percentage where x and y are dependent variables.

• Rate: Used for comparison:

• 1 event in 2 different groups

• 2 events in the same process

• 1 event in the same process but at 2 different times

• Intrinsic Rate: A proportion or a ratio.

• Units for measuring binary variables include 3 types: ratio, proportion, and rate.

# 2. Describing Disease Measures:

• Uses 2 concepts: prevalence and incidence.

• At-risk population: Includes all individuals: those who have the disease, those who don’t have the disease, and those who can’t get the disease.

• Factors affecting prevalence:

• Increase:

• Duration of the disease

• New cases

• Migration of healthy people

• Immigration of people susceptible to the disease

• Improved healthcare conditions

• Increased lifespan

• Decrease:

• Duration of the disease

• Case fatality rate

• Migration of sick people

• Immigration of healthy people

• Improved treatment conditions

• High case fatality rate

• Significance of prevalence:

• Describes the burden of disease

• Develops intervention projects

• Determines sample size

# 3. Prevalence Index (P):

• There are 2 types of P: point and period.

• P reflects the scale of a health problem.

• For common attributes, 10^n is usually between 1-100.

• Cumulative incidence rate: Measures the frequency of new events occurring during the risk period and in a given period.

• Point prevalence: Measures the proportion of individuals with the disease within the risk population at a specific point in time.

# 4. Incidence Index (CI):

• Essentially a probability, the risk of individuals developing the disease in a given period.

• Ranges from 0 to 1.

• P = disease at 1 point / total population at 1 point

• CI = new cases in 1 period / number of non-diseased individuals in the risk population at the beginning

• Easy to identify new cases with diseases that don’t have a cure or continuous course.

• Easy to identify the population within the observed time period.

• CI resembles the unit of speed (people/year).

# 5. Incidence Density:

• Another name for incidence rate.

• IR = new cases in the period / total risk time (person-time)

• Significance: Evaluates the effectiveness of interventions, detects disease outbreaks.

# 6. Attack Rate:

• A variant of the incidence rate applied to a limited population and short duration.

• AR = new cases / risk population at the start

• Secondary attack rate: new cases / number of contacts with those already infected.

# 7. Units for Measuring Mortality Frequency:

• Includes:

• Mortality rate

• Specific mortality rate

• Case fatality rate

• Case fatality ratio

• CDR = deaths in the period / average population of the period

• When calculating CDR based on mortality statistics, the sample is usually the average population at the midpoint of the period.

• Specific mortality rate: Includes types: age, gender, cause.

• Highest mortality rate from accidents and injuries in 2015: traffic accidents, drowning, suicide.

• Case fatality rate: Special because death doesn’t need to be part of the sample, as the disease already existed.

• DCR = deaths from a disease in the period / new cases in the same period

• Case fatality ratio reflects the severity of the disease.

# 8. Measures Identifying the Association of Risk Factors (RF) with Disease:

• Includes:

• Risk ratio (relative risk) – RR

• Odds ratio

• Attributable risk

• RR = incidence rate in exposed population / incidence rate in unexposed population

• RR = 1: No association.

• RR > 1: Positive association, further factors needed to identify RF.

• RR < 1: Negative association, further factors needed to identify RF.

• RR = IRe/IR0 = CIe/CI0

• Odds ratio: Commonly used in case-control studies.

• OR = (a/c) / (b/d)

• OR approaches RR when:

• The number of individuals in the control group increases

• The % of exposure in the disease and control groups remain constant.

• OR = 1: The odds of disease and no disease are equivalent.

• OR > 1: The odds of disease are higher than no disease.

• OR < 1: The odds of disease are lower than no disease.

• Odds: The ratio of the probability of an event occurring / the probability of the event not occurring.

• Attributable risk: Measures association and impact (in practice: measures impact more).

• There are 2 types of attributable risk: exposed group and unexposed group.

• Attributable risk for the exposed group: Indicates that not all new cases are due to exposure.

• New cases in the exposed group = new cases not due to exposure + new cases due to exposure

• AR = new cases in exposed – new cases in unexposed

• AR = 0: No link between exposure and disease.

• AR > 0: Positive link between exposure and disease.

• AR < 0: Negative link between exposure and disease.

• AR is also known as: excess risk, absolute risk.

• Percentage of attributable risk in the exposed group: Used to estimate what % of disease in the exposed group is attributed to the specific effect of exposure.

• AR% = 1 – 1/RR

• (CI0 – CIe ) / CIe = AR%

• Attributable risk for the population: Caused by RF in the entire population.

• PAR = incidence in the whole population – incidence in the unexposed

• PAR% = Pe x (RR-1) / 1+ Pe x (RR -1)

• IT = Pe x I(Te) + Po x I(To)

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