PU520: Principles of Epidemiology Unit 4 Epidemiologic Measurements Flashcards
(38 cards)
What are important mathematical terms applied to epidemiologic measures which are all types of ratios?
Rate, proportion (includes risk), and percentage.
Ratio as well.
What is defined as the value obtained by dividing one quantity by another?
How is it expressed?
Ratio
Ratios are sometimes expressed as percentages.
Ratio = X/Y
Example l: With respect to AIDS mortality, the sex ratio of deaths (male to female deaths) = X/Y, where: X = 450,451 and Y = 89,895. The sex ratio = 450,451/89,895 = 5 to 1 (approximately).
What is a type of ratio in which the numerator is part of the denominator?
How is it expressed?
Proportion
Proportions may be expressed as percentages.
Proportion is expressed as follows: proportion = A/A+B
Example 1: Proportion of AIDS deaths Suppose that
A = the number of male deaths from AIDS
A = 450,451
B = the number of female deaths from AIDS
B = 89,895
The proportion of deaths that occurred among males = 450,451/(450,451 + 89,895) = 0.83
What is a proportion that has been multiplied by 100?
How is it expressed?
Percentage
(A/A + B) x 100
Example 1: The percentage of male deaths from AIDS was (0.83 × 100) = 83%.
How can proportion (including percentage) be helpful in describing how important a health outcome is?
Because it looks at a health outcome and measures the cases across the population in which they occur.
For example, if 10 dorm residents get mononucleosis, we would want to know how large of a problem these 10 cases represent. If the dorm houses 20 total residents, that is 50% and seems pretty important.
While if the population of the dorm is 500, these cases would represent 2% of the population which paints an entirely different picture.
Clearly, these two scenarios paint a completely different picture of the magnitude of the problem. In this situation, expressing the count as a proportion is indeed helpful. In most situations, it will be informative to have some idea about the size of the denominator. Although the construction of a proportion is straightforward, one of the central concerns of epidemiology is to find and enumerate appropriate denominators to describe and compare groups in a meaningful and useful way.
How does rate (also a type of ratio) differ from a proportion because the denominator involves a measure of time?
Rate
Epidemiologic rates are composed of a numerator (the number of events such as health outcomes), a denominator (a population in which the events occur), and a measure of time. This measure of time is the time period during which events in the numerator occur. The denominator consists of the average population in which the events occurred during this same time period.
What are rates used to measure in epidemiology? (2)
To measure risks associated with exposures and provide information about the speed of development of a disease.
They are also used to make comparisons among populations.
T/F Medical publications may use the terms ratio, proportion, and rate without strict adherence to the mathematical definitions for these terms. Hence, you must be alert regarding how a measure is defined and calculated.
True
What types of information does epidemiologic measures provide? (3)
- the frequency of a disease or condition
- associations between exposures and health outcomes
- strength of the relationship between an exposure and a health outcome
How is the numerator in epidemiologic measures typically defined?
- Case definition (condition) - For epidemiologic measures to be valid, the case of disease or other health phenomenon being studied must be defined carefully and in a manner that can be replicated by others
- Frequency - How many cases are there
- Severity - Some epidemiologic measures employ morbidity as the number and others use mortality
How is the denominator in epidemiologic measures typically defined?
Does the measure make use of the entire population or a subset of the population?
Some measures use the population at risk, defined as those members of the population who are capable of developing a disease, for example, people who are not immune to an infectious disease.
- Existing (all cases) versus new cases.
What is the simplest and most frequently performed quantitative measure in epidemiology?
A count and it refers merely to the number of cases of a disease or other health phenomenon being studied.
The reader with the impression that counts, because they are simple measures, are of little value in epidemiology; this is not true, however. In fact, case reports of patients with particularly unusual presentations or combinations of symptoms often spur epidemiologic investigations. In addition, for some diseases even a single case is sufficient to be of public health importance.
For example, if a case of smallpox (now eradicated) or Ebola virus disease were reported, the size of the denominator would be irrelevant. That is, in these instances a single case, regardless of the size of the population at risk, would stimulate an investigation.
What does the term incidence refer to?
The number of instances of illness commencing, or of persons falling ill, during a given period in a specified population.
More generally, the number of new health-related events in a defined population within a specified period of time.
What are the different ways to express incidence?
Incidence rate, cumulative incidence, incidence density, and attack rate.
How is incidence rate defined and expressed?
The rate at which new events occur in a population.
The new events are usually new cases of disease but can be other health outcomes. The incidence rate is a rate because a time period during which the new cases occur is specified and the population at risk is observed.
The incidence rate denotes a rate formed by dividing the number of new cases that occur during a time period by the average number of individuals in the population at risk during the same time period times a multiplier.
When calculating incidence rate, why is it important to always choose an average number for the denominator when talking about a population during a specific time?
The midpoint of the population during the timeframe is chosen because of migration and how often populations change.
To overcome this challenge, the population at the midpoint of the year is used as the denominator and is considered to be the average population at risk.
When is an incidence rate called a cumulative incidence (incidence proportion)?
When ALL individuals in the population are at risk throughout the time period during which they were observed.
An example of a population in which all members of the population are at risk is a fixed or closed population (such as the participants in a cohort study) in which no new members are allowed to enter the study after it begins.
Here is a hypothetical calculation example for cumula-tive incidence: An epidemiologist studies cardiovascular disease among 23,502 male middle-aged alumni of an Ivy League university. Initial medical examinations certify that the alumni have never had a heart attack in the past. During the first year of the research, 111 alums have heart attacks.
Cumulative incidence = 111/23,502 = .005 (or if a percentage, 0.5%)
What is a variation of an incidence rate that is used when the time periods of observation of the members of a population vary from person to person?
Incidence density.
During a study that takes place over an extended period of time (for example, a cohort study, which is described later in the text), participants may be observed for varying periods of time because some drop out before the study is completed.
In order to make use of all participants’ data, we calculate incidence density according to the formula shown in the box. The numerator is the number of new cases during the time period and the denominator is the total person-time of observation.
Person-time is the total period of time that each individual at risk has been observed. For example, one person-year means that one subject has been observed for one year.
What is a type of incidence rate used when the occurrence of disease among a population at risk increases greatly over a short period of time?
Attack Rate
This is often related to a specific exposure, and is frequently used to describe the occurrence of foodborne illness, infectious diseases, and other acute epidemics.
An attack rate is not a true rate because the time dimension is often uncertain.
What, expressed as a proportion, refers to the number of existing cases of a disease or health condition, or deaths in a population at some designated time divided by the number of persons in that population?
What are the two types of this term?
PrevALence
Point prevalence - refers to all cases of a disease, health condition, or deaths that exist at a particular point in time relative to a specific population from which the cases are derived.
For example, if we are referring to illness (morbidity) in a group of people, the formula for point prevalence is shown in the following box.
Point prevalence = # ill at a point in time / total # in the group at a point in time
Point prevalence may be expressed as a proportion formed by dividing the number of cases that occur in a population by the size of the population in which the cases occurred times a multiplier. Note that point prevalence is a proportion and not a rate. If the value of 100 is used as the multiplier, prevalence becomes a percentage.
Period prevalence denotes the total number of cases of a disease that exist during a specific period of time (e.g., a week, month, year, or other interval).
An example of period prevalence is asthma period prevalence. The numerator for asthma period prevalence reflects whether a respondent answered affirmatively to the question “during the past 12 months, did anyone in the family have asthma?”
The time period for this measure is the past year.
Food For Thought
What does lifetime prevalence denote?
Lifetime prevalence denotes cases of disease diagnosed at any time during the person’s lifetime.
What does prevalence measures used to describe
Prevalence measures are used to describe the scope and distribution of health outcomes in the population.
The scope or amount of disease is called the burden of disease in the population.
By offering a snapshot of disease occurrence, prevalence data contribute to the accomplishment of two of the primary functions of descriptive epidemiology: to assess variations in the occurrence of disease and to aid in the development of hypotheses that can be followed up by analytic studies.
Populations that differ in size cannot be compared directly by using frequency data, i.e., just the numbers of cases. In order to make such comparisons, prevalence proportions need to be calculated. Then it is possible to compare the proportions of health outcomes among different geographic areas. See image.
How is incidence and prevalence interrelated concepts?
Prevalence of disease (P) is proportional to the incidence of the disease (I) times the duration of the disease (D).
P = ID
Consequently, when the incidence of a disease increases, the prevalence also increases.
What are other factors that cause prevalence of a disease to increase?
Its duration, in-migration of new cases, and development of treatments for the disease, including methods for extending the lives of patients who may not actually be cured.
An example of how the dura-tion of a disease affects its prevalence would be two diseases (A—long duration and B—short duration) that have similar incidence rates; we would expect disease A to have a higher prevalence than disease B. In the United States, the incidence of HIV infections has tended to remain constant over time and is much lower than HIV prevalence. Because more people with HIV infection are surviving for longer time periods, the prevalence of HIV is much greater than its incidence.
