STATISTICS AND RESEARCH DESIGN
Measures of frequency of disease Nikolaos Pandis, Associate Editor of Statistics and Research Design Bern, Switzerland, and Corfu, Greece
I
n observational studies, where the occurrence of a condition is of interest, we must initially define the study population. We often refer to the study population as the population at risk for developing the condition or the disease. In observational studies, the study population is defined through our research question and the choice of case definition. Once we have defined what constitutes the case and the study population, we can quantify the occurrence of the outcome of interest with reference to the size of the population at risk using the following 2 common types of measures of occurrence.1
1.
Prevalence is the number of existing cases (prevalent cases) divided by the number of persons in the study population at a defined point in time: number of cases in the study population at a specified time Prevalence 5 number of persons in the study population at a specified time
Prevalence can be interpreted as the probability that a certain person is a case at the defined time point; like all probabilities, it takes values from 0 to 1, or 0% to 100%. Prevalence is a measure of occurrence with no units; however, the point in time should be specified. The term “prevalence rate” is often incorrectly used instead of “prevalence.” Prevalence is a proportion and not a rate; therefore, those terms should not be confused. An example of prevalence is the proportion of people aged 20 to 30 years with confirmed periodontal disease in the study population at a defined time point. Prevalence is like a snapshot in time indicating how many have the condition and how many do not have the condition regardless of when the condition developed. For the prevalence of periodontal cases, we are not distinguishing old from new cases. Prevalence can be encountered either as point prevalence (as described Am J Orthod Dentofacial Orthop 2014;145:410-1 0889-5406/$36.00 Copyright Ó 2014 by the American Association of Orthodontists. http://dx.doi.org/10.1016/j.ajodo.2013.12.009
410
above) or as period prevalence, which answers the question of whether someone had the condition during a specified period of weeks, months, years, and so on. 2.
Incidence, in contrast to prevalence, quantifies the number of new cases (or incident cases) that develop in a population at risk during a specified time interval. Incidence is usually calculated by using incidence risk or incidence rate.
Incidence risk or cumulative incidence indicates the proportion of noncases in a population that subsequently becomes cases within a specified time interval: number of new cases at a specified time period Cumulative incidence 5 number of noncases at the beginning of that time period Cumulative incidence or incidence risk can be interpreted as the probability, or risk, that a subject will become a case during a specified period of time, and it usually applies to nonrecurrent events or to the first episode of an event. Again, incidence risk has no units, but as incidence risk increases with time, the time period should be specified. If we sum up the time that each participant was at risk of having the outcome, then we can arrive at the incidence rate, which indicates the number of new cases divided by the total person-time at risk during the period of interest: number of new cases at a specified time period Incidence rate 5 Sum of person-time at risk during that time period Incidence rate indicates the average rate or “speed” at which a person becomes a case in a given population. The incidence rate has units such as the number of cases per person-time. This could be cases per person-month, person-year, 1000 person-years, and so on, depending on what is more sensible for interpretation purposes. For example, if the number of new cases of periodontal
Pandis
411
Table. Measures of disease or condition occurrence and types of studies for which they apply Prevalence Burden of disease at a specified time point. Useful for planning health services.
Incidence Incidence indicates new cases for the period of interest and is generally preferred for studies looking at etiology of a condition.
Incidence rate If the population is relatively stable, and the condition is rare, then: incidence risk 5 incidence rate 3 period of follow-up. Incidence risk (or cumulative risk) is easier to interpret, but cannot be directly compared between studies with different follow-up durations. In a high turnover population and/or if the outcome of interest is common, incidence rate would be more suitable, since it takes into account the time periods for which subjects were under observation and remained at risk.
Cross-sectional studies
Cohort studies
Cohort studies
The above measures are not applicable for case-control studies, as we will see in the article dealing with this design.1
disease in the 20 to 30-year age group is on average 10 persons in a population of 100 per year, then: Incidence rate 5
10 50:1 cases per person 100 1
years
51 case per 10 person
years
In other words, for every 10 persons followed over a 1-year period, we expect 1 new case. WHAT IS THE RELATIONSHIP BETWEEN PREVALENCE AND INCIDENCE?
The proportion of cases in the study population at a particular point in time (prevalence) is a function of the incidence (new cases in the study population for the specified time period) and the average duration of a condition. When the rates of a disease are stable, the following formula relating incidence with prevalence applies: Prevalence5Incidence Duration of disease Since prevalence increases with increasing duration of disease, it is possible to find populations having the same disease incidence with a different prevalence because of variations in disease duration. For example, since the incidence of periodontal disease might decrease in one population and increase or remain stable in another, the prevalence can be higher in the first population because the subjects in the first population live longer (while still having periodontal disease).
Therefore, in a developed country where prevention of periodontal disease is more systematic, the relative number of new cases can decrease, but if the life expectancy were longer, this results in increased prevalence. This concept creates a paradox because the prevalence of a disease increases even after the introduction of a preventive program. Therefore, the risk for a disease or condition and its causality are more appropriately inferred from incidence; nevertheless, prevalence is important as an indicator of the burden of a disease and will allow for correct planning of health services. Scenarios for using the different types of measures of disease or condition occurrence and the types of studies for which they are applicable are shown in the Table. KEY POINTS
1. 2.
3.
Frequency of disease is measured by using prevalence and incidence. Prevalence is useful in measuring the disease burden in a population of interest and hence for planning health care services. Incidence is more useful for examining the etiology of a disease or condition.
REFERENCE 1. Gordis L. Epidemiology. 4th ed. Philadelphia: Saunders Elsevier; 2009. p. 37-57.
American Journal of Orthodontics and Dentofacial Orthopedics
March 2014 Vol 145 Issue 3
Measures of frequency of disease Nikolaos Pandis, Associate Editor of Statistics and Research Design Bern, Switzerland, and Corfu, Greece
I
n observational studies, where the occurrence of a condition is of interest, we must initially define the study population. We often refer to the study population as the population at risk for developing the condition or the disease. In observational studies, the study population is defined through our research question and the choice of case definition. Once we have defined what constitutes the case and the study population, we can quantify the occurrence of the outcome of interest with reference to the size of the population at risk using the following 2 common types of measures of occurrence.1
1.
Prevalence is the number of existing cases (prevalent cases) divided by the number of persons in the study population at a defined point in time: number of cases in the study population at a specified time Prevalence 5 number of persons in the study population at a specified time
Prevalence can be interpreted as the probability that a certain person is a case at the defined time point; like all probabilities, it takes values from 0 to 1, or 0% to 100%. Prevalence is a measure of occurrence with no units; however, the point in time should be specified. The term “prevalence rate” is often incorrectly used instead of “prevalence.” Prevalence is a proportion and not a rate; therefore, those terms should not be confused. An example of prevalence is the proportion of people aged 20 to 30 years with confirmed periodontal disease in the study population at a defined time point. Prevalence is like a snapshot in time indicating how many have the condition and how many do not have the condition regardless of when the condition developed. For the prevalence of periodontal cases, we are not distinguishing old from new cases. Prevalence can be encountered either as point prevalence (as described Am J Orthod Dentofacial Orthop 2014;145:410-1 0889-5406/$36.00 Copyright Ó 2014 by the American Association of Orthodontists. http://dx.doi.org/10.1016/j.ajodo.2013.12.009
410
above) or as period prevalence, which answers the question of whether someone had the condition during a specified period of weeks, months, years, and so on. 2.
Incidence, in contrast to prevalence, quantifies the number of new cases (or incident cases) that develop in a population at risk during a specified time interval. Incidence is usually calculated by using incidence risk or incidence rate.
Incidence risk or cumulative incidence indicates the proportion of noncases in a population that subsequently becomes cases within a specified time interval: number of new cases at a specified time period Cumulative incidence 5 number of noncases at the beginning of that time period Cumulative incidence or incidence risk can be interpreted as the probability, or risk, that a subject will become a case during a specified period of time, and it usually applies to nonrecurrent events or to the first episode of an event. Again, incidence risk has no units, but as incidence risk increases with time, the time period should be specified. If we sum up the time that each participant was at risk of having the outcome, then we can arrive at the incidence rate, which indicates the number of new cases divided by the total person-time at risk during the period of interest: number of new cases at a specified time period Incidence rate 5 Sum of person-time at risk during that time period Incidence rate indicates the average rate or “speed” at which a person becomes a case in a given population. The incidence rate has units such as the number of cases per person-time. This could be cases per person-month, person-year, 1000 person-years, and so on, depending on what is more sensible for interpretation purposes. For example, if the number of new cases of periodontal
Pandis
411
Table. Measures of disease or condition occurrence and types of studies for which they apply Prevalence Burden of disease at a specified time point. Useful for planning health services.
Incidence Incidence indicates new cases for the period of interest and is generally preferred for studies looking at etiology of a condition.
Incidence rate If the population is relatively stable, and the condition is rare, then: incidence risk 5 incidence rate 3 period of follow-up. Incidence risk (or cumulative risk) is easier to interpret, but cannot be directly compared between studies with different follow-up durations. In a high turnover population and/or if the outcome of interest is common, incidence rate would be more suitable, since it takes into account the time periods for which subjects were under observation and remained at risk.
Cross-sectional studies
Cohort studies
Cohort studies
The above measures are not applicable for case-control studies, as we will see in the article dealing with this design.1
disease in the 20 to 30-year age group is on average 10 persons in a population of 100 per year, then: Incidence rate 5
10 50:1 cases per person 100 1
years
51 case per 10 person
years
In other words, for every 10 persons followed over a 1-year period, we expect 1 new case. WHAT IS THE RELATIONSHIP BETWEEN PREVALENCE AND INCIDENCE?
The proportion of cases in the study population at a particular point in time (prevalence) is a function of the incidence (new cases in the study population for the specified time period) and the average duration of a condition. When the rates of a disease are stable, the following formula relating incidence with prevalence applies: Prevalence5Incidence Duration of disease Since prevalence increases with increasing duration of disease, it is possible to find populations having the same disease incidence with a different prevalence because of variations in disease duration. For example, since the incidence of periodontal disease might decrease in one population and increase or remain stable in another, the prevalence can be higher in the first population because the subjects in the first population live longer (while still having periodontal disease).
Therefore, in a developed country where prevention of periodontal disease is more systematic, the relative number of new cases can decrease, but if the life expectancy were longer, this results in increased prevalence. This concept creates a paradox because the prevalence of a disease increases even after the introduction of a preventive program. Therefore, the risk for a disease or condition and its causality are more appropriately inferred from incidence; nevertheless, prevalence is important as an indicator of the burden of a disease and will allow for correct planning of health services. Scenarios for using the different types of measures of disease or condition occurrence and the types of studies for which they are applicable are shown in the Table. KEY POINTS
1. 2.
3.
Frequency of disease is measured by using prevalence and incidence. Prevalence is useful in measuring the disease burden in a population of interest and hence for planning health care services. Incidence is more useful for examining the etiology of a disease or condition.
REFERENCE 1. Gordis L. Epidemiology. 4th ed. Philadelphia: Saunders Elsevier; 2009. p. 37-57.
American Journal of Orthodontics and Dentofacial Orthopedics
March 2014 Vol 145 Issue 3
