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By Professor Omar Hasan Kasule Sr.
Learning Objectives:
· Definitions of rates, hazards, ratios, and proportions
· Crude rates, specific rates, and standardized/adjusted rates.
Key Words and Terms:
· Denominator
· Numerator
· Parameter
· Proportion
· Rate, adjusted rate
· Rate, crude rate
· Rate, incidence rate
· Rate, specific rate
· Rate, standardized rate
· Statistic
Unit Outline
RATES
A. Definition
B. Crude Rates
C. Specific Rates
D. Adjusted /Standardized Rates
E. Standardization
PROPORTIONS
A. Definition of a Proportion
B. Examples of Proportions
UNIT SYNOPSIS
DEFINITIONS
Data can be summarized using parameters, computed from populations, or statistics, computed from samples. Discrete data is categorized in groups and can be qualitative or quantitative. It can be categorized before summarization. The main types of statistics used are measures of location such as rates, hazards, ratios, and proportions and measures of spread. Measures of location indicate accuracy or validity. Measures of spread or variation, such as variance and range, indicate precision.
RATES
A rate is the number of events in a given population over a defined time period and has 3 components: a numerator, a denominator, and time. The numerator is included in the denominator. The incidence rate of disease is defined as a / {(a+b) t} where a = number of new cases, b = number free of disease at start of time interval, and t = duration of the time of observation.
A crude rate for a population assumes homogeneity and ignores subgroups differences. It is therefore un-weighted, misleading, and unrepresentative. Inference and population comparisons based on crude rates are not valid.
Rates can be specific for age, gender, race, and cause. Specific rates are more informative than crude rates but are cognitively difficult to internalize, digest, and understand so many rates or be able to reach some conclusions.
An Adjusted or standardized rate is a representative summary that is a weighted average of specific rates free of the deficiencies of both the crude and specific rates.
Standardization eliminates the ‘confusing’ or ‘confounding’ effects due to subgroups. Standardization can be by direct standardization, indirect standardization, and life expectancy or regression techniques.
Both direct and indirect standardization involve the same principles but use different weights. Direct standardization is used when age-specific rates are available and indirect standardization is used when age-specific rates are not available. Both direct and indirect standardization use a ‘standard population’ which can be a combination of the two or more populations being compared, use of just one of the comparison populations as a standard for the others, using the national population, and using the world population.
Life expectancy is a form of age-standardized standardized mortality rate. Regression techniques provide a means of simultaneous adjustment of the impact of various factors on the rate.
PROPORTIONS
A proportion is the number of events expressed as a fraction of the total population at risk without a time dimension. The formula of a proportion is a/ (a+b) and the numerator is part of the denominator. Examples of proportions are: prevalence proportion, neonatal mortality proportion, and the perinatal mortality proportion. The term prevalence rate is a common misnomer since prevalence is a proportion and not a rate. Prevalence describes a still/stationary picture of disease. Like rates, proportions can be crude, specific, and standard.