Chapter 3- Measures of morbidity and mortality used in Epidemiology

Important to know about the fixed population in cumulative incidence

because the population is fixed; -no one is allowed to enter the denominator after the start of the observation -numerator can include only individuals who were members of that fixed population

Incidence density denominator

becomes person-time of observation

to determine the period prevalence

combine the number of existing cases at the beginning of the time interval with the new cases that occur during the interval

What is general fertility rate used for

comparisons of fertility among age, racial, and socioeconomic groups

Attack rate denominator

consist of all people whether well or ill, who were exposed to the agent during a time period

Attack rate numerator

consists of people who are ill as a result of the exposure to the suspected agent

incidence rate (cumulative incidence)

describes the rate of development of a disease in a group over a certain time period

Incidence rates

designation of a time period (week, month, year, etc) -to determine, one must be able to specify the date of onset of the condition during the time period

fetal death rate used to

estimate the risk of death of the fetus associated with the stages of gestation

Neonatal morality rate reflects

events happening after birth, primarily, congenital malformations, prematurity

the most general form of ratio

has no specified relationship between numerator and denominator

Maternal Mortality Rate reflects

health care access and socioeconomic factors, includes maternal deaths resulting from causes associated with puerpedioum (period after childbirth), eclampsia, and hemorrhage

Attack Rate formula

ill/(ill+well) x 100 during a time period

Infant mortality rate used for

international comparisons; a high rate indicates unmet health needs and poor environmental conditions

Proportion

is a measure that states a count relative to the size of the group -can demonstrate the magnitude of a problem

The reference population

is the population from which cases of a disease have been taken from -denominator in rates

cumulative incidence

is the proportion of a fixed population that become diseased during a stated period

One of the central concerns of epidemiology related to proportion

is to find and enumerate appropriate denominators to compare groups in a meaningful and useful way -however for some diseases a single case is enough to be a public health importance

Direct Method

may be used if age-specific death rates in a population to be standardized are known and suitable standard population is available

Interpretation of SMR at 2.0

means that the death rate in the study population is 2x greater then expected

General fertility rate

number of live births expected in a year per 1,000 women aged 15 to 49 years

Incidence rate calculation

number of people who develop the disease in a specified period/number of persons at risk of developing the disease in the same time period

Interpretation of SMR at 1.0

observed and expected numbers would be the same, indicating that observed mortality is not unusual

Crude rate

observed differences in rates may be the result of systematic factors (ex: sex or age distributions) within the population rather than true variation in rates

The denominator for incidence rates

population at risk -Individuals who already have the disease are excluded -Individuals who are also not capable of developing the disease are also excluded ex: Ovarian Cancer, Prostate Cancer

If duration of disease is short and incidence is high

prevalence becomes similar to incidence

if duration of disease is long and incidence is low

prevalence increases greatly relative to incidence

interpretation of prevalence

provides an indication of the extent of a health problem

Specific Rates

refer to a particular subgroup of the population defined in terms of race, age, sex, or single cause of death or illness

Adjusted rates

summary measures of the rate of morbidity and mortality in a population in which statistical procedures have been applied to remove the effect of differences in composition of various populations

Crude rate

summary rates based on the actual number of events in a population over a given time period

Person-year are derived by what in incidence density?

summing up the product of each category of length of observation and the number of subjects in the category

What is required in cumulative incidence

that the disease status be determined for everyone in the denominator

the numerator in rates consists of what?

the frequency of a disease over a specified period of time

The incidence rates

the frequency of the new cases in the numerator -Individuals who have a history of the disease are not included

definition of prevalence

the number of existing cases of disease or health condition in a population at some designated time

definition of incidence

the number of new cases of a disease that occur in a group during a certain time period

Successful treatment programs that would shorten the duration of a disease primarily affect

the prevalence of the disease

to calculate the rate

two periods of time are involved: -The beginning of the period -The end of the period

Crude Birth rate formula

# of live births within a given period/ population size at the middle of that period X 1,000 population

Formula for Incidence Density

# of new cases during the time period/ total person-time of observation

formula for Incidence density when observation is measured in years

# of new cases during the time period/ total person-years of observation

The basic concepts of rates can be broken down into:

-Crude rates -Specific rates -Adjusted Rates

Uses of Prevalence

-Describing the burden of a health problem in a population. -Estimating the frequency of an exposure. -Determining allocation of health resources such as facilities and personnel.

The 2 methods of adjusted rates

-Direct method -Indirect method

Population at risk may include:

-Those exposed to disease agent -un-immunized -may consist of the entire population

Examples of Counts

-Traffic fatalities in Manhattan in a 24-hour time period -college dorm students who had hepatitis -Foreign-born stomach cancer patients

Properties of Proportion

-a type of ratio in which the numerator is part of the denominator -it is important to know the size of the denominator -may be expressed as a percentage

Important factors of adjusted rates

-age is a common factor -age is probably the most important variable in risk of morbidity and mortality -rates can be adjusted for other variables

rates contain the following elements...

-disease frequency -unit size of population -time period during which an event occurs

Indirect method

-may be used in age-specific death rates of the population for standardization are unknown or unstable -Standardized mortality ratio (SMR) can be used to evaluate the results

Difficulty with cumulative incidence

-most of the regions we live and work contain dynamic population -leads to loss of follow up

risks versus rates

-one solution to the problem of geographic mobility and loss to follow up; is to use rates as an indicator of risks -groups with high rates of disease are at greater risk than are groups with low rates of the disease

Incidence rate contain 3 elements

1. numerator= the number of new cases 2. denominator= the population at risk 3. time= the period during which the cases occur

of 1,000 motorcycle fatalities, 950 of the victims are men and 50 are women. Find the sex ratio

950/50= 19:1 male to female

period prevalence

= # of people ill/ average population during a time period

point prevalence

= # of people ill/total # in the group at point in time

Ratio may be expressed as:

=x/y

fetal death rate

A proportion that compares the number of intermediate and/or late fetal deaths to the total number of live births and intermediate or late fetal deaths during the same period of time

Direct method insures that

Any observed difference that remain are not due to factors such as -age -race -sex

Applications of incidence Data

Help in research on the etiology/causality of disease -used to estimate the risks of developing the disease and the effects of exposure to a hypothesized factor of interest

perinatal mortality

Number of deaths from 28 weeks' gestation to one 7 days of life per 1000 total births

Specific rates vs crude rates

Specific rates are a much better indicator of risk than crude rates

Neonatal mortality rate

The death rate among newborns in the first 28 days of life.

Age-Specific Rate

The number of cases per age group population (during a specified time period)

Maternal Mortality Rate

The number of women who die giving birth per 100,000 births

infant mortality rate

The percentage of children who die before their first birthday within a particular area or country.

Interrelationship between prevalence and incidence

The prevalence of a disease is proportional to the incidence rate times the duration of a disease P=ID

Count

The simplest and most frequently performed quantitative measure in epidemiology -refers to the number of cases of a disease or other health phenomenon being studied

period prevalence

The total number of cases of a disease that exist during a specified period of time -could be a week, month, or longer time interval

The definition of rate

a ratio that consists of a numerator and a denominator and in which time forms part of the denominator

Incidence Density

an incidence measure used when members of a population or study group are under observation for different lengths of time -different length of time due to; death, drop out, migration, etc

Ratio

Defined as the value obtained by dividing one quantity by another -proportions, rates, and percentages are also ratios -consists of a numerator and a denominator

Proportional Mortality Ratio

Indicates relative importance of a specific cause of death; not a measure of risk of dying of a particular cause

proportion equation

P=(a)/(a+b)x 100

To provide a direct estimate of the risk of developing a disease

This is a use primarily for incidence data

crude birth rate

Used to measure the population growth -used as an index to compare developed and developing countries -its higher in less developed countries than developed countries

the denominator in rates involves what?

a measure of time