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CHAPTER 4: DESCRIPTIVE EPIDEMOLOGY Cheat Sheet by

Descriptive epidemiology involves observation, definitions, measurements, interpretations, dissemination, of health-related states or events by person, place and time.

Descri­ptive Study Design

Descri­ptive study design include case reports and case series, cross-­sec­tional surveys, and explor­atory ecologic designs.

Descri­ptive Studies

ECOLOGIC STUDIES -
involves making compar­­isons between variables where the unit of analysis is aggregated data on the population level rather than on the individual level
CROSS-­­SE­CTION STUDIES
all variables measured at a point in time
CASE REPORT
is a profile of a single indivi­dual; it includes qualit­ative descri­ptive research of the facts in chrono­logical order
CASE SERIES
involves a small group of patients with a similar diagnosis

RATIOS, PROPOR­TIONS, RATES

 
Ratios, propor­tions, and rates are commonly used measures for describing dichot­omous data. The general formula for a ratio, propor­tion, or rate is: X/Y x 10z
RATES
a type of frequency measure where the numerator involves nominal data that represent the presence or absence of a health­-re­lated state or event
RATIOS
the values of x and y are distinct, such that the values of x are not contained in y. The rate base for a ratio is 100 = 1
PROPOR­TIONS
x is contained in y. A proportion is typically expressed as a percen­tage, such that the rate base is 102 = 100.
Dichot­omous data- Divided or dividing into two parts or classi­fic­ations.

CALCUL­ATION RATES

DEFINI­TIONS
CALCUL­ATIONS
Incidence rate- is the number of new cases of a specified health­-re­lated state or event reported during a given time interval
Incidence Rate= New cases occurring during a given time period­/po­pul­ation at risk during the same time period multiplied by 10z
Mortalilty Rate- is the total number of deaths reported during a given time
Mortality Rate = Deaths occurring during a given time period/ Population from which deaths occurred Multiplied by 10z
Person­-Time Rate- When the denomi­nator of the incidence rate is the sum of the time each person was observed
Person Time rate= New cases occurring during an observ­ati­onp­eri­od/Time each person observed, totaled for all persons multiply by 10z
Attack Rate- It involves a specific population during a limited time period, such as during a disease outbreak. It is also referred to as a cumulative incidence rate or risk
Attack Rate=New cases occurring during a shirt time period­/Po­pul­ation at risk at the beginning of the time period multiplied by 100
Secondary Attack Rate- the rate of new cases occurring among contacts of known cases.
SAR= New cases among contacts of primary cases during a short time period­/(P­opu­lations at beginning of time period)- (primary cases) multiplied by 100
Point Preval­ence- he frequency of an existing health­-re­lated state or event during a time period.
Point Preval­ence= Existing cases of a disease or event at a point in time/total study population at a point in time multiplied by 100
 

STRENGTHS AND WEAKNESSES

 
DESCRI­PTION
STRENGTHS
WEAKNE­SSESS
Ecologic study
Aggregate data involved (not for specific indivi­duals
Take advantage of preexi­sting data, can be used to evaluate programs, polices or regual­tions implemted at the ecological level
Suscep­tible to confou­nding exposure and disease or injury outcomes not measured on the same indivi­duals
Cross-­section studies
All variables measured at a point in time no distin­ction between potential risk factors and outcomes
Control over study population and measur­ements. several associ­ations between variables can be studied at the same time, short time period required
Potential bias from low response rate, higher proportion of long-term survivors, does not yield incidence or relative risk
Case study
A snapshot descri­ption of a problem or situation for an individual or group
in-depth descri­ption, provides clues to identify a new disease or adverse health effect resulting from exposure or experience
Conclusion limited to the indivi­dual, group,and or context under study, cannot be used to establish a cause -effect relati­onship

CRUDE RATES VS AGE-AD­­JUSTED RATE

Crude rate is calculated without any restri­ctions such as age. however, these rates are limited if the epidem­iol­ogist is trying to compare them between subgroups of the population or over time because of potential confou­nding influe­nces, such as differ­ences in the age distri­bution between groups.
An age-ad­justed rate is a weighted average of the age-sp­ecific rates.R­ates based on data covering age intervals of 5 or 10 years are generally preferred because they are more stable than rates based on single­-year age intervals.
Indirect method of age adjust­ment- In situations in which age-sp­ecific rates are unstable because of small or missing numbers, age adjustment is still possible with the indirect method.
standard morbid­ity­/mo­rtality ratio (SMR).
Interp­ret­ation ■   SMR = 1: The health­-re­lated states or events observed were the same as expected from the age-sp­ecific rates in the standard popula­tion. ■   SMR > 1: More health­-re­lated states or events were observed than expected from the age-sp­ecific rates in the standard popula­tion. ■   SMR < 1: Fewer health­-re­lated states or events were observed than expected from the age-sp­ecific rates in the standard popula­tion.
 

4 TYPES OF DATA

Nominal
unordered categories or classes (e.g., gender, race/e­thn­icity, marital status, occupa­tion).
Ordinal
additional inform­ation provided by the order among categories (e.g., stage or grade of cancer).
Dsicrete
integers or counts that differ by fixed amounts, with no interm­ediate values possible (e.g., number of new cases of lung cancer reported in the United States in a given year, number of children, number of sick days taken in a month).
Continous
measurable quantities not restricted to taking on integer values (e.g., age, weight, temper­ature).

TABLES, GRAPHS AND NUMERICAL MEASURES

The simplest table is the frequency distri­bution, which is a complete summary of the freque­ncies, or number of times each value appears.
Epidemic Curve- is a histogram that shows the course of an epidemic by plotting the number of cases by time of onset. ■ A stem-a­­nd­-leaf plot is a display that organizes data to show their distri­bution.
A histogram shows a frequency distri­bution for discrete or continuous data.
Bar charts are often used for graphi­cally displaying a frequency distri­bution that involves nominal or ordinal data.

Numerical Methods

Measures of central tendency refer to ways of design­ating the center of the data. The most common measures are the arithmetic mean, geometric mean, median, and mode
Arithmetic and Geometric Mean
Measures of disper­sion, also called the spread or variab­ility, are used to describe how much data values in a frequency distri­bution vary from each other and from the measures of central tendency.

Numerical Methods

Measures of central tendency refer to ways of design­ating the center of the data. The most common measures are the arithmetic mean, geometric mean, median, and mode
Arithmetic and Geometric Mean
Measures of disper­sion, also called the spread or variab­ility, are used to describe how much data values in a frequency distri­bution vary from each other and from the measures of central tendency.
 

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  • Published: 4th October, 2022

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