FINALS: MEASURES OF DISEASE FREQUENCY AND MORBIDITY Flashcards
Number of diseases, injury, death, and etc.
Frequency
Compare one part of distribution to another part of the distribution or to entire population
Frequency
are used to describe how common an illness (or other health event) is with reference to the size of the population (the population at risk) and a measure of time
MEASURES OF DISEASE FREQUENCY
Numerator
Count of events
Denominator
Population size
Disease
Morbidity
Death
Mortality
Birth
Natality
Epidemiologic Measures Categorized as
Ratio, Proportion, or Rate
Frequently Used Measures of Morbidity
Incidence proportion (or attack rate or risk)
Secondary attack rate
Incidence rate (or person-time rate)
Point prevalence
Period prevalence
Number of cases of a particular health event
Count
Examples:
□ Number of college students who smoke
□ Vehicular fatalities in Davao City during a 24-hour period
Count
Division of one quantity by another
Ratio
Includes any expression with a numerator and a denominator (general term)
Ratio
Examples:
□ Community X has:
■ 600,000 males: 400,000 females
■ 600:400 or 3:2
Ratio
A ratio in which the numerator is contained in the denominator
Proportion
RATIO
• Can be used as _________ (male: female ratio).
descriptive
RATIO as _________ (risk ratio, rate ratio, odds ratio)
Analytic tools
• Commonly used epidemiologic ratio.
DEATH-TO-CASE RATIO
Number of deaths attributed to a particular disease during a specified period divided by number of new cases of that disease identified during the same period.
DEATH-TO-CASE RATIO
Measure of severity of illness.
DEATH-TO-CASE RATIO
Tells us what fraction of the population is affected
Proportion
Proportion may be expressed as
Expressed as a percentage, decimal, or fraction
A ratio representing change over time
Rate
Describes how quickly disease occurs in a population
Rate
Can be expressed as a percentage or per unit time
Rate
X and Y can be any number, including ratios.
Ratio (R)
Type of ratio where the numerator is usually a count, and the denominator is a time elapsed.
Rate (r)
Type of ratio where the numerator is part of the denominator.
Proportion (p)
A proportion is multiplied by 100.
Percent (P)
• Proportion of persons who are unaffected at the beginning of a study period, but who experiences a risk event during the study period
Risks
Constitutes the people at risk for the event at the BEGINNING of the study period
Cohort
The probability of an event that will occur (P) divided by the probability that the event will not occur (1-P)
Odds
Refers to the presence of disease in a population
Morbidity
Two categories of measurement to describe frequency of disease in a specified population
Incidence and prevalence
new cases
Incidence
existing cases – old and new
Prevalence
2 measure of incidence
Incidence Proportion
Incidence Rate
2 measured of prevalence
Point Prevalence
Period Prevalence
• State of being symptomatic or unhealthy for a disease / condition.
• Presence of disease in a population
MORBIDITY
Two categories of measurement to describe Morbidity frequency of disease in a specified population
• Incidence
• Prevalence
Of you have 2 types of morbidity
Comorbidity
○ count of persons with a specific trait or condition
○ count of persons using a program or intervention dorm. aa sa sa meeting? ○ count of positive or negative events
Numerators
Sources of numerator data:
○ surveys, registries, telephone book ○ researcher’s own counts or observations
○ in the context of a formal study
○ total population in an area over a defined period of time
○ it is the reference population in which health events are drawn and measured
Denominators
Sources of denominator data:
○ census
○ population statistics
• Occurrence of new cases of events (disease, injury, death, etc.) in a population at risk over a specified period of time.
INCIDENCE/INCIDENT CASES
Number of transitions (well-ill, uninjured-injured, alive-dead).
INCIDENCE/INCIDENT CASES
Types of incidence
• Incidence Proportion/ Risk
• Incidence Rate/ Person-Time Rate
Number of newly diagnosed cases
Incidence
Incidence can be measured using:
□ Counts = incident cases / incidence
□ Element of time = incidence rate
□ Proportion = cumulative incidence
Rate is a measure of disease risk
□ Transition from a non-diseased state to a diseased state
INCIDENCE RATE
In incidence rate, the numerator reflects the number of ________ during the time period
new cases
In incidence rate there are two types of denominators
Person at risk
Person-time
In incidence rate (denominator): person at risk is ______ population
Closed
In incidence rate (denominator): person time is ______ population
Open/ dynamic
INCIDENCE RATE FOR DYNAMIC POPULATION Also known as
incidence density
• The proportion of initially disease-free population that develop new cases of disease in a specified period of time.
INCIDENCE PROPORTION/RISK
Measures probability that healthy people will develop disease during a specified time.
INCIDENCE PROPORTION/RISK
Synonyms of incidence proportion/ risk
Attack rate
Incidence Risk
Probability of developing disease
Cumulative Incidence
Criteria needed for cumulative incidence
o Knowledge of health status of the study population (diseased or not)
o Time of onset (acute vs. chronic)
o Specification of numerator
o Specification of population “at risk”
AKA cumulative incidence / incidence proportion
Usually applied to outbreak settings
ATTACK RATES
Total number of new cases / total population
Overall attack rate
Number of persons who ate a specific food item and became ill / total number of persons who ate the food
Food specific attack rate
Number of persons exposed to the risk factor for developing a disease within the incubation period following exposure to the primary case
Secondary attack rate
Risk of getting the disease during a specified period (duration of an outbreak).
Attack rate
Total number of new cases divided by total population.
Overall attack rate
Number of persons who ate and became ill divided by total number of persons who ate the food.
Food specific attack rate
Number of cases among contacts of primary cases divided total number of contacts.
Secondary attack rate
□ Not a measure of risk
□ Usually expressed as a percentage (multiplier = 100)
□ Used to express burden of disease (new + old cases)
Prevalence
Number of persons in a defined population who have a specified disease or condition at a given point in time
Prevalence
▣ Proportion of persons with a particular disease or attribute on a particular date
Point prevalence
▣ Proportion of persons with a particular disease or attribute at any time during the interval
Period prevalence
Answers the question how many people have had the disease at any point during a certain time period?
Period prevalence
“Do you currently have asthma?”
Point prevalence
“Have you had asthma during the last(n] years?”
Period prevalence
“Have you ever had asthma?”
Cumulative incidence
Number of affected persons present in the population at a specific time divided by the number of persons in - old cases new the population at that time (new + old cases)
PREVALENCE RATE
Expressed as number of existing cases per X population
PREVALENCE RATE
Visit every household in the community, using interviews or physical examinations to get a count of those affected
PREVALENCE RATE
IMPORTANCE OF PREVALENCE
KEY POINTS:
▣ Used by health planners for determining workload
▣ For monitoring control programs for chronic conditions
▣ If incidence data not available, can use prevalence to estimate importance of disease
Proportion/rate or persons who develop a condition during a particular time period.
Incidence
Proportion of persons who have condition at/during particular time period
Prevalence
New cases only (Numerator)
Incidence
New and pre-existing cases (Numerator)
Prevalence
Lists the clinical criteria by which public health professionals determine whether a person’s illness is included as a case in an outbreak investigation.
Case definition
Helps decide who to count in your numerator.
Helps to differentiate who might have similar but not exactly the same symptoms.
Case definition
The frequency (number) of new occurrences of disease, injury, or death-that is, the number of transitions from well to ill, from uninjured to injured, or from alive to dead—in the study population during the time period being examined.
Incidence (incident cases)
The number of persons in a defined population who had a specified disease or condition at a particular point in time, usually the time a survey was done.
Point prevalence (prevalent cases)
The number of persons who had a specified disease at any time during a specified time interval. Period prevalence is the sum of the point prevalence at the beginning of the interval plus the incidence during the interval. Because period prevalence combines incidence and prevalence, it must be used with extreme care.
Period prevalence
The frequency (density) of new events per per-son-time (e.g., person-months or person-years). Incidence density is especially useful when the event of interest (e.g., colds, otitis media, myocardial infarction) can occur in a person more than once during the period of study.
Incidence density
A clearly defined group of persons who are studied over a period of time to determine the incidence of death, disease, or injury.
Cohort
The proportion of persons who are unaffected at the beginning of a study period, but who undergo the risk event (death, disease, or injury) during the study period.
Risk
The frequency (number) of new events that occur in a defined time period, divided by the average population at risk. Often, the midperiod population is used as the average number of persons at risk (see Incidence rate).
Rate
A rate calculated as the number of incident cases (see above) over a defined study period, divided by the population at risk at the midpoint of that study period. Rates of the occurrence of births, deaths, and new diseases all are forms of an incidence rate.
Incidence rate
The proportion (usually expressed as a percentage of a population that has a defined disease or condition at a particular point in time. Although usually called a rate, it is actually a proportion.
Prevalence rate
: Rates that apply to an entire population, with no reference to characteristics of the individuals in the population. Crude rates are generally not useful for comparisons because populations may differ greatly in composition, particularly with respect to age.
Crude rates
: Rates that are calculated after a population has been categorized into groups with a particular characteristic. Examples include age-specific rates and gender-specific rates. Specific rates generally are needed for valid comparisons.
Specific rates
: Crude rates that have been modified (adjusted) to control for the effects of age or other characteristics and allow for valid comparisons of rates.
Standardized (adjusted) rates
: The preferred method of standardization if the specific rates come from large populations and the needed data are available.
Direct standardization
: The method of standardization used when the populations to be compared are small (so that age-specific death rates are unstable) or when age-specific death rates are unavailable from one or more populations but data concerning the age distribution and the crude death rate are available. Here standard death rates (from the standard population) are applied to the corresponding age groups in the different population or populations to be studied. The result is an “expected” (standardized crude) death rate for each population under study. These “expected” values are those that would have been expected if the standard death rates had been true for the populations under study. Then the standardized mortality ratio is calculated.
Indirect standardization
The observed crude death rate divided by the expected crude death rate.
Standardized mortality ratio (SMR)
