Principles of Epidemiology. Frequency Measures Used in Epidemiology

1. Author: Dr. Obumneke Amadi: Transcript
Title: Frequency Measures Usedin Epidemiology
The strengths and limitations of the epidemiologic study design In “A Multistate
Outbreak of Cyclosporiasis case study”. The epidemiologic studies include case-control studies
and cohort studies and they are used to prove the association between a disease and its causative
factors.
The strengths of the epidemiological methodology include the following:
 Epidemiologic study uses the scientific method in the generating and testing of
hypotheses.
 Epidemiological study is cheaper in terms of material or human resources to
conduct
 It provide generalizable results in a shorter time
 It has the ability to offer positive interventions that solve problems
The limitations of epidemiologic study results are factors such as bias, misclassification and
measurement error, confounding, and effect modification (Barnett and Hyman, 2006):
 Bias is deviation of results in a mistaken estimate of an exposure’s effect on the
risk of disease on the design and analysis of a study.
 Misclassification is an error caused by inaccuracy in measuring exposures or
disease status. Which can lead to the observed association to appear either
stronger or weaker than normal
 Confounding is an overestimation, underestimation or change in direction of the
effect of an exposure with the effect of another variable that is associated with the
exposure and is an independent risk factor for the disease e.g. “consider a study

2. that found an association between drinking alcoholic beverages and lung cancer”
(Barnett and Hyman, 2006)
 Effect modification occurs when the effect of the risk factor on an outcome can
vary, depending on the value of another variable
(Barnett and Hyman, 2006)
The use of rates, ratios, and proportions as it relates to the epidemiologic calculations.
Three kinds of frequency measures are used with two-category variables (also called
dichotomous variables). These frequency measures are Ratios, Proportions, and Rates. All three
measures are based on the same formula: i.e. Ratio, proportion, and rate = x/y x 10ⁿ (MODULE
IV, n.t):
A. A ratio is used to compare the occurrence of a variable in two different groups. These
may be two completely independent groups, or one may be included in the other. For
example, we could compare the age of men attending smoking in either of the following
ways: young or old( MODULE IV, n.t)
B. Rates measure the occurrence of an event in a population over time and is used to
calculate the number of cases occurring during a given time period over the population at
risk during the same time period. They reflect groupings of people based on time, place
and person. E.g. (MODULE IV, n.t):
 Time: month, week, day or hour
 Place: country, state, school, institution
 Person: age, sex

3. C. Proportions are usually expressed as percentages. Proportions are usually expressed as
percentages, so the value of the constant (10ⁿ) = 10² = 100: 0.24 X 100 = 24 (24%)
(MODULE IV, n.t).
The occurrence of disease in one group can be well compared with another E.g. males with
females; one county with another. All the three frequency measures are used to calculate in the
same way i.e. a ratio compares two independent groups, a proportion compares one group with a
larger one to which it belongs, and a rate to measure an event in a population over time. In
infectious disease epidemiology they are used to describe morbidity/disease and mortality/death
(MODULE IV, n.t).
Reference
MODULE IV (n.t) Statistical Measures: Principles of Infectious Disease
Epidemiology. Retrieved from health.mo.gov/training/epi/Mod4StudentOutline.pdf
Merrill, R. (2011). Principles of epidemiology: Workbook. Sudbury, MA: Jones & Bartlett
Pg.67-72