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Closed populations

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"enclosure" Also called cohorts Epidemiologic cohort ≡ a group of individuals followed over time They gain no new members after they are established and lose members only when members die or are no longer at risk of becoming a case for whatever reason. they begin with a certain number of individuals and shrink over time as mortality takes its toll and individuals are removed from risk.

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Open populations

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open populations add new members through birth and immigration, and lose members through emigration and death. Over time, open populations may grow, remain the same size, or shrink, depending on their rate of inflow or outflow An open population that is in a steady state— so, for example, as one person dies a new individual is born into the population and another ages up to replace the death— is said to be stationary Inflow (immigration, births) Outflow (emigration, death) An open population in "steady state" (constant size and age) is said to be stationary

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Incidence Proportion 1/3

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The incidence proportion (cumulative incidence, average risk) of a disease is a measure of disease occurrence in which the numerator consists of the number of disease onsets that occurred during the period of observation and the denominator consists of the number of individuals at risk in the closed population as of the beginning of follow-up: number of onsets during study interval/no at risk at beginning of study interval Incidence proportions can only be measured in cohorts, and cannot be calculated in open populations. The denominator includes only individuals at risk of developing the condition being studied and therefore excludes individuals who are not capable of developing the condition under consideration. Ex: in studying uterine cancer, the denominator excludes women who had already experienced uterine cancer, women with prior hysterectomies, and men, because these individuals are not capable of developing the condition being studied.

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Incidence Proportion 2/3

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An incidence proportion is the average risk of developing the condition under consideration for the period of observation. the terms incidence proportion and risk are used interchangeably in epidemiology. Since the incidence proportion represents an accumulation of new cases over time, it is also referred to as cumulative incidence

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Incidence Proportion 3/3

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To interpret an incidence proportion properly, the length of the time at risk must be specified. Characteristics of the population should be made clear. Ex: the incidence proportion (risk) of breast cancer in American women. The lifetime risk for this outcome is 12% (1 in 8). In contrast, the risk in women between the ages of 60 and 69 is 3.5% (1 in 29). Finally, the risk between ages 50 and 59 is 2.4% (1 in 42). Our understanding of population characteristics and the length of follow-up should temper our interpretation of an incidence proportion.

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Incidence Rate 1/2

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The incidence rate ( IR; incidence density) of a disease is the number of disease onsets divided by the sum of person-time in the population: no. of onsets/ sum of person time at risk A person-time unit is the amount of time a person is observed during the study. One person observed for one year contributes one person-year to the denominator. One person observed for two years accounts for two person-years. Two people observed for one year each also accounts for two person-years (and so on).

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Incidence Rate 2/2

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Person-time is counted only when a person is at risk of being detected as a case. Person-time is no longer counted after: the person develops the disease under investigation the person withdraws from the study, or the study ends.

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How to Interpret Incidence Rates?

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Firstly, incidence rates represent the speed , rapidity , density or intensity at which populations are expected to generate cases. For example, a rate of 5 per 100 person-years is expected, on the average, to generate 5 cases in 100 people followed for one year. Secondly, incidence rates reflect the incidence proportion (risk) of the disease when the disease is ''rare'' according to the formula: Risk ≈ Rate × Time. Ex: a rate of 1 per 100 person-years over a one-year period corresponds to a one-year risk ≈ Rate × Time = ( 0.01 per person − year ) × ( 1 year ) = 0.01 or 1% Thirdly, the incidence rate in a population is related to its survival experience.

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Incidence Rate (IR) in Closed Populations

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no. of onsets/ sum of person-time at risk Synonyms: incidence density, person-time rate Interpretation A: "Speed" at which events occur in a population Interpretation B: When disease is rare: rate per person-year ≈ one-year average risk Calculated differently in closed and open populations

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Incidence Rates in Open Populations

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Rates can also be estimated in open populations with this formula: onsets/ avg population size x duration of observation In open populations that are rapidly increasing or decreasing in size, it is common to use the population size mid-way through the period of observation as an estimate of the average population size. .

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Prevalence

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Prevalence ( prevalence proportion, point prevalence) refers to the proportion of individuals in a population that have a disease or condition at a specific point in time: no. old and new cases/ no. of people The numerator of a prevalence calculation includes all individuals with the condition under consideration regardless of when the disease commenced. The denominator is the total number of individuals under consideration. Prevalence can be calculated in both open and closed populations.

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How to Interpret Prevalence?

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Prevalence simply refers to the proportion of individuals in the population currently with a disease or condition. When based on a simple random sample from a population, the prevalence is an estimate of the probability that an individual currently has the condition in question. The prevalence of a disease in a population depends on the rate of inflow of cases into the population and outflow of cases from the population. Inflow is determined by the incidence rate of the disease in the population and the immigration into the population of people who already have the disease.

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Prevalence

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Outflow is determined by the rate of resolution either through recovery or death, and also by the emigration of cases from the population. The level of water in the basin represents the prevalence of the condition. The prevalence of a condition can increase from either an elevation in incidence or decreases in the death rate. Ex: improved survival of HIV/AIDS patients through effective treatment will increase the prevalence of the condition in the population if the incidence of HIV/AIDS remains constant. Thus, the prevalence of disease is related to the duration of the disease according to this formula: Prevalence ≈ (incidence rate) × (average duration of disease) Ex: a disease with an incidence rate of 0.01 year − 1 and average duration of 1/2 year under steady-state conditions has prevalence ≈ 0.01 year − 1 × 0.5 year = 0.005.

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Incidence vs. Prevalence

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Incidence and prevalence represent distinct measures of disease frequency. Incidence addresses the transition from the disease-free state to the diseased state. In contrast, prevalence addresses current health. Thus, because it is linked to the duration of illness, prevalence is not as well suited as incidence for studying causation.

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Measures of Association

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Exposure (E) an explanatory factor or potential health determinant; the independent variable Disease (D) the response or health-related outcome; the dependent variable Measure of association (syn. measure of effect) any statistic that measures the effect on an exposure on the occurrence of an outcome

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Arithmetic (αριθμός) Comparisons

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Measures of association are mathematical comparisons Mathematic comparisons can be done in absolute terms or relative terms Let us start with this ridiculously simple example: I have $2 You have $1

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Absolute Comparison

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Absolute comparisons are derived by subtraction and using (original units of measure kg) In absolute terms, I have $2 MINUS $1 = $1 more than you Note: the absolute comparison was made with subtraction

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Relative Comparison

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Relative comparisons are derived by division (the division cancels out units, making a unit-free comparison} Recall that I have $2 and you have $1. In relative terms, I have $2 ÷ $1 = 2 times as much as you

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Relative and Absolute Comparisons

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Ex: an exposed group demonstrates a rate of 2 per 100 person-years, while a nonexposed group demonstrates a rate of 1 per 100 person-years. In absolute terms, the exposure increased the rate by (2 case per 100 person-years) − (1 case per 100 person-years) = 1 case per 100 person-years. This rate difference represents the effect of the exposure in absolute terms . Alternatively, we may say that the exposed group has 2 per 100 person-years (twice) the rate of the nonexposed. This rate ratio represents the effect of the exposure in relative terms

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Absolutes Comparisons Applied to Risks

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Suppose, I am exposed to a risk factor and have a 2% risk of disease. You are not exposed and you have a 1% risk of the disease. In absolute terms, I have 2% MINUS 1% = 1% greater risk of the disease This is the risk difference

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Relative Comparisons Applied to Risks

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In relative terms I have 2% ÷ 1% = 2 twice your risk This is the relative risk associated with the exposure

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Relative and Absolute Comparisons

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Absolute comparisons are made by subtraction. The rate or risk difference ( RD ) quantifies the effect of an exposure in absolute terms according to this formula: RD=R1-R0 where R1 represents the risk or rate of disease in the exposed group and R0 represents the risk or rate in the nonexposed group. This formula may also be applied to prevalence ''rates,'' in which case it describes a prevalence difference . Positive RDs indicate the excess rate associated with exposure in absolute terms. Negative RD s indicate the deficit in the rate or risk.

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Relative Measures of Effect (1/3)

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The rate or risk ratio ( RR ) quantifies the effect of an exposure in relative terms: Let RR represent the rate or risk ratio RR= R1/R0 where R1 ≡ the risk or rate in the exposed group R0 ≡ the risk or rate in the non-exposed group

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Relative Measures of Effect (2/3)

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The ratio of two incidence rates is a rate ratio. The ratio of two incidence proportions is a risk ratio. When this formula is applied to the ratio of two proportions, it results in a prevalence ratio. All of these ratio measures of effect are referred to as relative risks . Interpretation: The RR quantifies the excess ( RRs greater than 1) or deficit ( RR s less than 1) in the rate or risk of disease associated with exposure in relative terms. It is literally the risk multiplier associated with exposure.

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Relative Measures of Effect (3/3)

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Ex: an RR of 2 indicates that the exposure doubles the rate or risk of disease, while an RR of 1/2 indicates that the exposure cuts the rate or risk in half. Thus, the RR indicates both the direction and strength of an observed association. RRs greater than 1 indicate a positive association; those less than 1 indicate a negative association. The further the RR gets from 1, the stronger the association. Ex: an RR of 3 indicates a stronger positive association than an RR of 2. Analogously, an RR of 1/3 indicates a stronger negative association than an RR of 1/2

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Epidemiologic Measures

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In order to study the distribution and determinants of a certain outcome, it is first necessary to measure its frequency. Several mathematical measures are used to convey information about the occurrence of disease. Counts Proportions Ratios Rates

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Measures of Disease Occurrence

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Counts Proportions Ratios Rates

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Counts

answer

A count refers to the number of cases of the disease or health-related event under investigation. For epidemiologic purposes, counts are not very informative without knowing the population size Cannot make comparisons between populations based on counts Useful for health resources allocation (personnel and services needed to treat the sick people) within each population

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Proportion

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Proportion= A/ A+B = Number of people with disease (A)/ total number of population Takes a numeric value between 0 and 1 Can be expressed as a percentage A proportion indicates the fraction of a population that is affected with a certain condition Persons included in the numerator are always part of the denominator

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Ratio

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In a ratio, the numerator and the denominator are independent (they have no specific relationship) i.e. persons included in the numerator are not part of the denominator

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Rate

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A rate is a ratio that incorporates the element of TIME Includes the following components: Disease frequency (numerator) Population size Time period during which an event occurs E.g. 11 cases of tuberculosis per 100,000 persons per year.

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Epidemiologic Measures

answer

the frequency or events and conditions in populations the effects of an exposure the potential impact of an intervention.

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Measures of Disease Frequency

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Measures of disease frequency quantify how often a disease or condition occurs within a given population. measures of disease frequency are also called measures of occurrence . The three main measures of disease frequency are: Incidence proportion (risk) Incidence rate (incidence density) Prevalence

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Measures of Disease Frequency(1)

answer

All three of these measures of disease frequency are types of ratios consisting of a numerator and denominator. The numerator of each measure of disease frequency is some type of count of cases. The denominator is a measure of population size or ''person-time.''

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Closed populations

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Closed populations: Also called cohorts They gain no new members after they are established and lose members only when members die or are no longer at risk of becoming a case for whatever reason. they begin with a certain number of individuals and shrink over time as mortality takes its toll and individuals are removed from risk.

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Closed Population ≡ Cohort

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Cohort (Latin cohors, meaning "enclosure" Epidemiologic cohort ≡ a group of individuals followed over time

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Open populations(1)

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open populations add new members through birth and immigration, and lose members through emigration and death. Over time, open populations may grow, remain the same size, or shrink, depending on their rate of inflow or outflow An open population that is in a steady state— so, for example, as one person dies a new individual is born into the population and another ages up to replace the death— is said to be stationary . An open population in "steady state" (constant size and age) is said to be stationary

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Incidence

answer

Refers to the development of NEW cases of a disease during a defined period of time in previously disease-free individuals who are at risk of developing the disease (i.e. capable of developing the disease) Incidence quantifies the development of disease, i.e., the transition from a non-diseased to diseased state Provides an estimate of the probability (or risk) that a person will develop a disease during a specified period of time Central to study causal factors of diseases

question

Incidence Proportion

answer

the number of disease onsets that occurred during the period of observation and the denominator consists of the number of individuals at risk in the closed population as of the beginning of follow-up: IP= no. of onsets during study interval/ no. of risk at beginnning of study interval

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Incidence Proportion(1)

answer

Incidence proportions can only be measured in cohorts, and cannot be calculated in open populations. The denominator includes only individuals at risk of developing the condition being studied and therefore excludes individuals who are not capable of developing the condition under consideration. Ex: in studying uterine cancer, the denominator excludes women who had already experienced uterine cancer, women with prior hysterectomies, and men, because these individuals are not capable of developing the condition being studied.

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Incidence Proportion(2)

answer

An incidence proportion is the average risk of developing the condition under consideration for the period of observation. the terms incidence proportion and risk are used interchangeably in epidemiology. Since the incidence proportion represents an accumulation of new cases over time, it is also referred to as cumulative incidence Synonyms for incidence proportion Attack rate Risk Probability of developing disease Cumulative incidence In the outbreak setting, the term attack rate is often used as a synonym for risk. Attack rate= is the risk of getting the disease during a specified period, such as the duration of an outbreak.

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Incidence Proportion (Attack Rate)

answer

A variety of attack rates can be calculated. Overall attack rate is the total number of new cases divided by the total population. A food-specific attack rate is the number of persons who ate a specified food and became ill divided by the total number of persons who ate that food, as illustrated in the previous potato salad example.

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Incidence Proportion(3)

answer

To interpret an incidence proportion properly, the length of the time at risk must be specified. Characteristics of the population should be made clear. Ex: the incidence proportion (risk) of breast cancer in American women. The lifetime risk for this outcome is 12% (1 in 8). In contrast, the risk in women between the ages of 60 and 69 is 3.5% (1 in 29). Finally, the risk between ages 50 and 59 is 2.4% (1 in 42). Our understanding of population characteristics and the length of follow-up should temper our interpretation of an incidence proportion.

question

Incidence Rate

answer

The incidence rate ( IR; incidence density) of a disease is the number of disease onsets divided by the sum of person-time in the population: IR= no. of onsets/ sum of person-time @ risk A person-time unit is the amount of time a person is observed during the study. One person observed for one year contributes one person-year to the denominator. One person observed for two years accounts for two person-years. Two people observed for one year each also accounts for two person-years (and so on).

question

Incidence Rate(1)

answer

Person-time is counted only when a person is at risk of being detected as a case. Person-time is no longer counted after: the person develops the disease under investigation the person withdraws from the study, or the study ends.

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IR(1)

answer

Incidence rate or person-time rate is a measure of incidence that incorporates time directly into the denominator. A person-time rate is generally calculated from a long-term cohort follow-up study, wherein enrollees are followed over time and the occurrence of new cases of disease is documented. Typically, each person is observed from an established starting time until one of four "end points" is reached: onset of disease, death, migration out of the study ("lost to follow-up"), or the end of the study.

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IR(2)

answer

Similar to the incidence proportion, the numerator of the IR is the number of new cases identified during the period of observation. However, the denominator differs. The denominator is the sum of the time each person was observed, totaled for all persons. This denominator represents the total time the population was at risk of and being watched for disease. the incidence rate is the ratio of the number of cases to the total time the population is at risk of disease. Firstly, incidence rates represent the speed , rapidity , density or intensity at which populations are expected to generate cases. Secondly, incidence rates reflect the incidence proportion (risk) of the disease when the disease is ''rare'' according to the formula: (Incidence Proportion) Risk ≈ Rate × Time Ex: a rate of 1 per 100 person-years over a one-year period corresponds to a one-year risk ≈ Rate × Time = ( 0.01 per person − year ) × ( 1 year ) = 0.01 or 1% Thirdly, the incidence rate in a population is related to its survival experience. The area under the curve in this diagram is equivalent to the person-time in the cohort.

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Incidence Rate (IR) in Closed Populations (cont)

answer

IR= no. onsets/ sum of person-time at risk Synonyms: incidence density, person-time rate Interpretation A: "Speed" at which events occur in a population Interpretation B: When disease is rare: rate per person-year ≈ one-year average risk Calculated differently in closed and open populations

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Incidence Rates in Open Populations(cont)

answer

Rates can also be estimated in open populations with this formula: IR= onsets/ avg population size x duration of observation In open populations that are rapidly increasing or decreasing in size, it is common to use the population size mid-way through the period of observation as an estimate of the average population size. .

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Prevalence(cont)

answer

Refers to EXISTING cases of disease or health condition in a population (generally irrespective of the duration of disease) Quantifies the burden or "magnitude" of disease Who has the disease and who doesn't regardless of when the disease developed (it could have developed yesterday, last month, last year, or 10 years ago) "Snapshot" of disease in the population PR= number of cases of a disease present in the population at a specified time/ number of persons in the population at that specified time

question

Prevalence(1)

answer

Prevalence ( prevalence proportion, point prevalence) refers to the proportion of individuals in a population that have a disease or condition at a specific point in time: prevalence= no. of old and new cases/ no. of people The numerator of a prevalence calculation includes all individuals with the condition under consideration regardless of when the disease commenced. The denominator is the total number of individuals under consideration. Prevalence can be calculated in both open and closed populations. Prevalence simply refers to the proportion of individuals in the population currently with a disease or condition. When based on a simple random sample from a population, the prevalence is an estimate of the probability that an individual currently has the condition in question.

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2 types of prevalence measures

answer

point prevalence period prevalence

question

Point Prevalence

answer

Number of existing cases /total population at a set point in time (e.g. September 1,2012) Example: On September 1, 2012, Community A had: Population= 1600 Existing cases of influenza= 29 Point prevalence of influenza= 29/ 1600 = 1.8%

question

Period Prevalence

answer

Number of existing cases /total population during a specified time period (e.g. Sep 1, 2012 to Sep 30, 2012) Number of cases= existing cases on Sep 1 (irrespective of disease duration), AND those newly diagnosed until Sep 30, 2012 Example: (refer to previous slide) Between Sep 1, 2012 and Sep 30, 2012, Community A has: Population= 1600 (let's assume population remains fixed) Existing cases of influenza on Sep 1, 2011= 29 New (Incident) cases of influenza between Sep 1 and Sep 30= 7 Period prevalence= (29+7)/ 1600= 0.0225= 2.25%

question

Incidence vs. Prevalence

answer

Incidence and prevalence represent distinct measures of disease frequency. Incidence addresses the transition from the disease-free state to the diseased state. Focuses on New events Measures the risk of developing disease; the rate at which disease develops per year (or unit of time) in a population Think about it as the speedometer of a car (60 miles per hour) Needed to investigate disease etiology In contrast, prevalence addresses current health. Thus, because it is linked to the duration of illness, prevalence is not as well suited as incidence for studying causation. Focuses on EXISTING events Measure of the magnitude or "burden" of disease in a population Think about it as the odometer of a car (miles accumulated within a certain period) Very helpful in planning health services

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Interrelationship between Incidence and Prevalence

answer

Incidence and prevalence are interrelated When the incidence of a disease increases, its prevalence in the population also increases Consider 2 diseases A and B: They have the same incidence rate A has long duration and B has short duration Which disease has a higher prevalence? Answer= disease A When the duration of a disease increases, its prevalence increases

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How to Interpret Prevalence?

answer

The prevalence of a disease in a population depends on the rate of inflow of cases into the population and outflow of cases from the population. Inflow is determined by the incidence rate of the disease in the population and the immigration into the population of people who already have the disease. Outflow is determined by the rate of resolution either through recovery or death, and also by the emigration of cases from the population. The level of water in the basin represents the prevalence of the condition. The prevalence of a condition can increase from either an elevation in incidence or decreases in the death rate.

question

Ways to increase prevalence

answer

Increase incidence -> increase inflow Increase average duration of disease -> decreased outflow

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Interrelationship between Incidence and Prevalence

answer

Ex: improved survival of HIV/AIDS patients through effective treatment will increase the prevalence of the condition in the population if the incidence of HIV/AIDS remains constant. Thus, the prevalence of disease is related to the duration of the disease according to this formula: Prevalence ≈ (incidence rate) × (average duration of disease)

question

Factors that Influence Prevalence

answer

Factors that increase prevalence: - Increase in disease incidence - Longer duration of disease - Treatments that prolong the patient's life but do not cure the disease -In-migration of cases -In-migration of people at risk of developing the disease - Better diagnosis Factors that decrease prevalence: - Decrease in disease incidence - Shorter duration of disease - High death rate from disease - Treatments that cure disease - Out-migration of cases

question

Annual Mortality Rate

answer

=total number of deaths from all causes in 1 year/ number of persons in the population at mid year Represents the "risk" of dying from any cause in a given year Denominator: includes population from which deaths occurred Because the population changes over time, an approximation is used: Generally, we use the number of persons in the population at midyear Usually multiplied by 10n to get rid of decimals (e.g. multiplied by 103 and expressed per 1,000 population)

question

example

answer

Death rate in the US in the year 2005: Number of deaths in the US during 2005= 2,448,017 Population of the US on July 1, 2005 (midyear) = 296,410,404 Annual death rate in 2005: = 2,448,017/ 296,410,404 = 0.008259 = 8.259 per 1,000 (multiply by 103) = 825.9 per 100,000 (multiply by 105)

question

Mortality Rate

answer

Mortality rate does not have to be annual Can be calculated over 5 years, etc.. Time period is arbitrary BUT must be precisely specified

question

Case Fatality Rate

answer

=number of deaths due to certain disease during a time period/ total number of disease cases during same period Number of deaths due to a disease that occur among people who have that disease Measure of "lethality" and "severity" of a disease Links mortality to morbidity Usually multiplied by 100 and expressed as % Hantavirus infection: 45 cases of hantavirus infections occurred in a US state in the year 1993 22 cases out of 45 were fatal Case Fatality Rate (CFR): = (22/45)*100= = 48.9% Interpretation: approximately 49% of people who get a hantavirus infection die from it (deadly infection)

question

Proportionate Mortality Rate

answer

= mortality due to a specific cause during a time period/ mortality due to all causes during same period What proportion of all deaths is caused by a specific disease? Usually multiplied by 100 and expressed as %

question

Proportional Mortality Rate for Heart Disease in 2005

answer

Deaths due to heart disease in 2005/ total number of deaths in 2005 *100 = (652,091/2,448,017)*100 = 26.6% Interpretation: of all deaths that occurred in the US in 2005, 26.6% were caused by heart disease Note: If the PMR from heart disease increases (over time for e.g.), this does not mean that the risk of death from heart disease is increasing

question

Comparison of PMR and Mortality Rate

answer

Conclusion: PMR is not a measure of risk of dying from a certain cause Do not confuse it with the Mortality Rate for that cause If the PMR for heart disease increases over time, it does not necessarily mean that the risk of death from heart disease is also increasing It could mean that mortality from other causes (e.g. cancer) is decreasing and cancer is contributing less to overall mortality

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Infant Mortality Rate

answer

number of infant deaths among infants aged 0-365 days in a given year/ number of live births during same year Usually expressed per 1,000 live births Powerful indicator of health status of country

question

Years of Potential Life Lost (YPLL)

answer

Measure of premature mortality Age of death for every deceased person is subtracted from a predetermined "standard" age at death (e.g. 65 years in the US) YPLL for an infant dying at 1 year= (65-1)= 64 years YPLL for a man dying at 50= 15 year Younger age of death more years of potential life lost Total YPLL is calculated by adding all individuals' "years of potential life lost" for each cause of death

question

Different Forms of Rates

answer

Rates used to quantify morbidity and mortality can be of different forms: 1. Crude: Calculated for a population as a whole Does not take in account differences in population structure such as age and sex Unadjusted 2. Specific or Stratified: Calculated for specific subgroups within a population or for specific causes of death/ diseases: Cause-specific rate Age-specific rate Sex-specific rate Race-specific rate 3. Adjusted or Standardized: Statistically calculated rate that is adjusted according to a standard population

question

Cause-specific Rate

answer

number of deaths (or number of cases) due to a certain disease/ population size at midpoint of time period Usually multiplied by 100,000

question

Accidents-specific Mortality Rate

answer

number of deaths due to accidents in 2005/ population size in yr 2005 Number of deaths due to accidents= 117,809 Population size on July 1, 2005 (selected at midpoint of the year)= 296,410,404 Accidents-specific mortality rate = 117,809/296,410,404 = 0.000397 Or 39.7 per 100,000 Interpretation: 39.7 per 100,000 people in the US in the year 2005 died because of accidents

question

Age-specific Mortality Rate

answer

number of deaths within a certain age group/ number of people in the same age group (during same period) Usually multiplied by 100,000 Example: In the US in 2005, there were 1,717 deaths due to cancer in the age group 15-24 years. Number of people in the 15-24 years age group= 42,076,849. What's the age-specific cancer mortality rate? The age-specific cancer mortality rate in the 15-24 years age group is: (1,717/42,076,849)*105 = 4.1 per 100,000

Epidemiology Lecture 10 – Flashcards

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