Measures of Disease Frequency

Measures of Disease Frequency
Dr. Ume Sughra Assistant Professor MBBS, MPH, FCPS Al-Shifa School of Public Health

Overview Rate, Ratio & Proportion Summary of different measures
Measures of Morbidity Attack Rate Measures of Mortality

If you can measure that of which you speak, and can express it by a number, you know something of your subject, but if you cannot measure it, your knowledge is meager and unsatisfactory. -William Thomson, Lord Kelvin, engineer, mathematician and physicist ( )

Rate, Ratio & Proportion
Most basic measure Obtained by simply dividing one quantity by another without implying any specific relationship between the numerator and denominator, Male : Females (572/457=1.25:1). In ratio, the numerator & denominator are mutually exclusive (neither included in each other).

Rate, Ratio & Proportion contd..
A proportion is a type of ratio in which the numerator is included in the denominator. Usually expressed in Percentage. For example: The proportion of women over the age of 50 who have had a hysterectomy, or The number of fetal deaths out of the total number of births (live births plus fetal deaths). The proportion of males=572/1092 =0.52 (52%)

A rate is a proportion with specifications of time. It is a basic measure of disease frequency in which there is a distinct relationship between the numerator and denominator with a measure of time being an intrinsic part of the denominator. For example, the number of newly diagnosed cases of breast cancer per 100,000 women 2 years.

In epidemiology, demography, and vital statistics, a rate is an expression of the frequency with which an event occurs in a specified population. Rates are used for comparison of experiences between populations at different times, different places or among different classes of persons.

Components of a rate: Numerator Denominator Specified time Multiplier to convert the rate from an awkward fraction or a decimal to a whole number. Rate = No of events in a specified period * n Susceptible person time /average population at risk during that period

Summary of Different Measures
Ratio: A relationship between two numbers (x:y) Proportion: is a specific type of ratio in which numerator is included in the denominator and expressed as a %age. Rate is a special form of proportion which includes specification of time.

Measures of morbidity Sources of morbidity statistics
Disease reporting-communicable diseases, cancer registries Data accumulated as a by-product of insurance and prepaid medical care plans Group health and accident insurance Prepaid medical care plans State disability insurance plans Life insurance companies Hospital insurance plans-Blue Cross Railroad Retirement Board Armed Forces Veterans Administration

Measures of morbidity contd..
Tax-financed public assistance and medical care plans Public assistance, aid to the blind, aid to the disabled State or federal medical care plans Hospitals and clinics Absenteeism records-industry and schools Pre-employment and periodic physical examinations in industry and schools Case-finding programs Records of military personnel Morbidity surveys on population samples (e.g., National Health Survey, National Cancer Surveys)

Measures of morbidity Incidence:
Number of new cases of a disease that occur during a specified period of time in a population at risk for developing the disease

R Bonita, R Beaglehole, T Kjellstrom:Basic Epidemiology, ed 2
R Bonita, R Beaglehole, T Kjellstrom:Basic Epidemiology, ed 2. World Health Organization

Result has been multiplied by 1,000 so that we can express the incidence per 1,000 persons. Choice of 1,000 is completely arbitrary- could have used 10,000, 1 million, or any other figure. ‘New cases of disease’ Measure of events-the disease is identified in a person who develops the disease and did not have the disease previously. As incidence is a measure of events (i.e., transition from a nondiseased to a diseased state), incidence is a measure of risk.

Risk can be looked at in any population group, such as a particular age group, males or females, an occupational group, or a group that has been exposed to a certain environmental agent, such as radiation or a chemical toxin. Denominator of incidence represents the number of people who are at risk for developing the disease. Any individual who is included in the denominator must have the potential to become part of the group that is counted in the numerator.

If we are calculating incidence of uterine cancer: Denominator must include only women, because men would not have the potential to become part of the group that is counted by the numerator-that is, men are not at risk for developing uterine cancer. For incidence to be a measure of risk, must specify a period of time, and know that all of the individuals in the group represented by the denominator have been followed up for that entire period.

Choice of time period is arbitrary: could calculate incidence in 1 week, incidence in 1 month, incidence in 1 year, incidence in 5 years, and so on. Important point is that whatever time period is used in the calculation must be clearly specified. All individuals included in the calculation must have been observed (at risk) for the entire period.

Cumulative incidence: Incidence calculated using a period of time during which all of the individuals in the population are considered to be at risk for the outcome (measure of risk) Number of new cases of a disease during a given period of time Total population at risk

Provides an estimate of the probability, or risk, that an individual will develop a disease during a specified period of time. Assumes that the entire population at risk at the beginning of the study period has been followed for the specified time interval for the development of the disease/outcome.

Incidence rate (also called an incidence density): Measure of incidence in which the numerator is the number of new cases and denominator consists of the sum of the different times that each individual was at risk. Often expressed in terms of person-years. Number of new cases of disease during given time period Total person-time of observation

Figure: Identifying newly detected cases of a disease
Figure: Identifying newly detected cases of a disease. Step 1: Screening for prevalent cases at baseline. Downloaded from: StudentConsult (on 25 January :45 AM) © 2005 Elsevier

Figure: Identifying newly detected cases of a disease
Figure: Identifying newly detected cases of a disease. Step 2: Follow-up and rescreening at 1 year to identify cases that developed during the year. Downloaded from: StudentConsult (on 25 January :45 AM) © 2005 Elsevier

Prevalence: Number of affected persons present in the population at a specific time divided by the number of persons in the population at that time.

Example: How would we know the prevalence of arthritis in a certain community on a certain date? We might visit every household in that community and, using interviews or physical examinations, determine how many people have arthritis on that day. This number becomes the numerator for prevalence. The denominator is the population in the community on that date.

Difference between incidence and prevalence: Prevalence can be viewed as a slice through the population at a point in time at which it is determined who has the disease and who does not. In so doing, we are not determining when the disease developed. Some individuals may have developed arthritis yesterday, some last week, some last year, and some 10 or 20 years ago.

When we survey a community to estimate the prevalence of a disease, we generally do not take into account the duration of the disease. Therefore, the numerator of prevalence includes a mix of people with different durations of disease, and as a result we do not have a measure of risk. If we wish to measure risk, we must use incidence, because in contrast to prevalence, it includes only new cases or events and a specified time period during which those events occurred.

Point Prevalence: Prevalence of the disease at a point in time Number of cases found to be affected at this point in time population size at that point in time

Period Prevalence: Number of cases found to be affected at a starting point in time, plus the number of cases newly incident during a specified interval of time Population size (e.g., as of the starting time point or at midpoint of the interval) How many people have had the disease at any time during a certain period, such as during a single calendar year.

Some people may have developed the disease during that period, and others may have had the disease before and died or been cured during that period. Important point is that every person represented by the numerator had the disease at some time during the period specified. Period prevalence combines information about the point prevalence of the condition with information about newly incident cases.

Example: Let us consider incidence and prevalence. Figure shows five cases of a disease in a community in The first case of the disease occurred in 2003, and the patient died in 2004. The second case developed in 2004 and continued into The third case was a person who became ill in 2004 and was cured in 2004. The fourth case occurred in 2003, and the patient was cured in 2004. The fifth case occurred in 2003 and continued through 2004 and into 2005.

Figure: Example of incidence and prevalence: I.
Downloaded from: StudentConsult (on 25 January :45 AM) © 2005 Elsevier

For this example, we will consider only the cases (numerators) and will ignore the denominators. In this example, what is the numerator for incidence in 2004? We know that incidence counts only new cases. Because two of the five cases developed in 2004, the numerator for incidence in 2004 is 2.

Figure: Example of incidence and prevalence: II.
Downloaded from: Student Consult (on 25 January :45 AM) © 2005 Elsevier

What about the numerator for point prevalence? This depends on when we do our prevalence survey. If we do the survey in May, the numerator will be 4. If we do the survey in July, the numerator will also be 4.

If we do the survey in September, however, the numerator will be 3. If we do it in December, the numerator will be 2. Thus, the prevalence will depend on the point during the year at which the survey is performed.

Relation between Incidence and Prevalence: Incidence is a measure of risk and that prevalence is not, because it does not take into account the duration of the disease. There is an important relationship between incidence and prevalence: In a steady-state situation, in which the rates are not changing and in-migration equals out-migration, the following equation applies: Prevalence= Incidence X Duration of disease

Figure :The natural history of disease and some sources of data relating to each interval.

Figure: Relationship between incidence and prevalence: I.

Figure: Relationship between incidence and prevalence: II.

Figure: Relationship between incidence and prevalence: III.

Figure: Relationship between incidence and prevalence: IV.

Hypothetical example of a study of seven people over seven years

Incidence: Number of new events (3) divided by the sum of the lengths of time at risk of getting the disease for the population (33 person-years), i.e. 9.1 cases per 100 person years. Cumulative incidence: Number of new events in the population at risk (3) divided by the number of people in the same population free of the disease at the beginning of the period (7), i.e. 43 cases per 100 persons during the seven years.

Prevalence: Depends on the point in time at which the study takes place Case fatality rate: 33% representing 1 death out of 3 diagnosed cases.

Attack Rate contd.. A person who acquires the disease from that exposure (e.g., from a contaminated food) is called a primary case. A person who acquires the disease from exposure to a primary case is called a secondary case. Secondary attack rate is therefore defined as the attack rate in susceptible people who have been exposed to a primary case. Good measure of person-to-person spread of disease after the disease has been introduced into a population, and it can be thought of as a ripple moving out from the primary case.

Attack Rate Attack Rate: Food specific Attack Rate:

Attack Rate contd.. Often calculate the secondary attack rate in family members of the index case. Secondary attack rate also has application in noninfectious diseases when family members are examined to determine the extent to which a disease clusters among first-degree relatives of an index case, which may yield a clue regarding the relative contributions of genetic and environmental factors to the cause of a disease.

You do not die from being born, nor from having lived, nor from old age. You die from something…. There is no such thing as a natural death: Nothing that happens to a man is ever natural, since his presence calls the world into question. All men must die: but for every man his death is an accident and, even if he knows it and consents to it, an unjustifiable violation. Simone de Beauvoir, writing of her mother's death, in A Very Easy Death

Measures of mortality contd..
Mortality Rates: Annual death rate or mortality rate from all causes:

Age specific mortality rate:

Cause specific mortality rate: Age and cause specific mortality rate:

Case Fatality Rates: What percentage of people diagnosed as having a certain disease die within a certain time after diagnosis?

What is the difference between case-fatality rate and mortality rate? In the mortality rate, the denominator represents the entire population at risk of dying from the disease, including both those who have the disease and those who do not have the disease (but who are at risk of developing the disease). In the case-fatality rate, the denominator is limited to those who already have the disease. Case-fatality is a measure of the severity of the disease. Can also be used to measure any benefits of a new therapy.

Comparison of Mortality and Case-Fatality Rate: Assume a population of 100,000 people of whom 20 are sick with disease X, and in one year, 18 of the 20 die from disease X: The mortality rate in that year as a result of disease X : The case-fatality rate as a result of X:

Proportionate mortality: Proportionate mortality from cardiovascular disease in the United States in 1999:

Is a proportion, not a rate Represents the proportion of deaths caused by any disease out of the total deaths

Comparison of Mortality Rate and Proportionate Mortality: Deaths from Heart Disease in Two Communities, A and B Community A Community B Mortality rate from all causes 30/1,000 15/1,000 Proportionate mortality from heart disease 10% 20% Mortality rate from heart disease 3/1,000

Comparing mortality in different populations: Direct Age adjustment Indirect Age adjustment Direct age adjustment: A standard population is used in order to eliminate the effects of any differences in age between two or more populations being compared. A hypothetical "standard" population is created to which both the age-specific mortality rates from the early period and the age-specific mortality rates from the later period are applied.

By applying mortality rates from both periods to a single standard population, any possibility that observed differences could be a result of age differences in the population is eliminated. By applying each age-specific mortality rate to the population in each age group of the standard population, expected number of deaths that would have occurred had those rates been applied is derived. Then calculate the total number of deaths expected in the standard population had the age-specific rates of the early period applied and the total number of deaths expected in the standard population had the age-specific rates of the later period applied.

Dividing each of these two total expected numbers of deaths by the standard population, we can calculate an expected mortality rate in the standard population if it had had the mortality experience of the early period and the expected mortality rate for the standard population if it had the mortality experience for the later period Therefore, differences in age-composition of the population are no longer a factor.

A Hypothetical Example of Direct Age Adjustment: I. Comparison of Total Death Rates in a Population at Two Different Times Early Period Later period Population No. of Deaths Death Rate per 100,000 900,000 862 96 9oo,000 1,130 126

A Hypothetical Example of Direct Age Adjustment: II. Comparison of Age-Specific Death Rates in a Population in Two Different Times Early period Later period Age Group (yr) Population No. of Deaths Death Rate per 100,000 All ages 900,000 862 96 9oo,000 1,130 126 30-49 500,000 60 12 300,ooo 30 10 50-69 300,000 396 132 400,000 400 100 70+ 100,000 406 200,000 700 350

Total no. of deaths expected in the standard population: 2,238 1,830
A Hypothetical Example of Direct Age Adjustment: III. Carrying Out an Age Adjustment Using the Total of the Two Populations as the Standard Age Group (yr) Standard Population “Early" Age-specific Mortality Rates per 100,000 Expected No. of Deaths Using "Early" Rates” "Later" Age-specific Mortality Rates per 100,000 Expected No. of Deaths Using "Later" Rates All ages 1,800,000 30-49 800,000 12 96 10 80 50-69 700,000 132 924 100 700 70+ 300,000 406 1,218 350 1,050 Total no. of deaths expected in the standard population: 2,238 1,830 Age adjusted Rates:

Indirect age adjustment (Standardized Mortality Ratios): Often used when numbers of deaths for each age-specific stratum are not available. It is also used to study mortality in an occupationally exposed population

Example: Do people who work in a certain industry, such as mining, have a higher mortality than people of the same age in the general population? To answer the question of whether a population of miners has a higher mortality than we would expect in a similar population that is not engaged in mining, the age-specific rates for such a known population, such as all men of the same age, are applied to each age group in the population of interest.

This will yield the number of deaths expected in each age group in the population of interest, if this population had had the mortality experience of the known population. Thus, for each age group, the number of deaths expected is calculated, and these numbers are totaled. The numbers of deaths that were actually observed in that population are also calculated and totaled. The ratio of the total number of deaths actually observed to the total number of deaths expected, if the population of interest had had the mortality experience of the known population, is then calculated.

Standardized Mortality Ratio (SMR):

Computation of a Standardized Mortality Ratio (SMR) for Tuberculosis, All Forms (TBC), for White Miners Ages 20 to 59 Years, United States, 1950 Estimated Population for White Miners Death Rate (per 100,000) for TBC in Males in the General Population Expected Deaths from TBC in White Miners if They Had the Same Risk as the General Population Observed Deaths from TBC in White Miners Age (yr) (1) (2) (3) = (1) ×(2) (4) 20-24 74,598 12.26 9.14 10 25-29 85,077 16.12 13.71 20 30-34 80,845 21.54 17.41 22 35-44 148,870 33.96 50.55 98 45-54 102,649 56.82 58.32 174 55-59 42,494 75.23 31.96 112 Totals 534,533 181.09 436

SMR is calculated by totaling the observed number of deaths (436) and dividing it by the expected number of deaths (181.09), which yields a result of 2.41. Multiplication by 100 is often done to yield results without decimals. If this were done in this case, the SMR would be 241. SMR of 100 indicates that the observed number of deaths is the same as the expected number of deaths. SMR greater than 100 indicates that the observed number of deaths exceeds the expected number, SMR less than 100 indicates that the observed number of deaths is less than the expected number.

Acknowledgements Prof. James C Anthony (Michigan State University)
Dr. Arslan Mazhar (Health Services Academy) Dr. Saima Hamid (Health Services Academy)

References Leon Gordis. Epidemiology, ed 3.Philadelphia, Elsevier Saunders, 2004. R Bonita, R Beaglehole, T Kjellstrom:Basic Epidemiology, ed 2. World Health Organization. CH Hennekens, JE Buring, SL Mayrent eds. Epidemiology in Medicine. Philadelphia, Lippincott Williams and Wilkins 1987.

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