Epidemiology
Validity specificity
defined as the ability of the test to identify correctly those who do not have the disease.
Validity sensitivity
defined as the ability of the test to identify correctly those who have the disease.
Active surveillance
denotes a system in which project staff are recruited to carry out a surveillance program.
Passive surveillance
denotes surveillance in which available data on reportable diseases are used, or in which disease reporting is mandated or requested, with the responsibility for the reporting often falling on the health care provider or district health officer.
How can you tell if a study had internal validity?
if the inferences (i.e., an estimate, an association, etc.) from the study population reflect the inferences that would be observed in the target population.
Cumulative incidence
is a proportion that is 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 (it is a measure of risk)
Morbidity
refers to the state of being diseased or unhealthy within a population and refers to an incidence of ill health in a population.
Two components of validity
sensitivity and specificity
etiology
the cause or causes of a disease
Underlying cause of death
the disease or injury which initiated the train of morbid events leading directly or indirectly to death or the circumstances of the accident or violence which produced the fatal injury.
Crude mortality rate
the mortality rate from all causes of death for a population during a specified time period. The denominator is the population at the mid-point of the time period. For example, the crude mortality rate for Missouri in 2003 was 896 deaths per 100,000 people.
Incidence rate of a disease
the number of new cases of a disease that occur during a specified period of time in a population at risk for developing the disease. (The critical element in defining incidence rate is NEW cases of disease.)
Attack Rate
the number of people exposed to something (a suspect food, a disease, etc.) who became ill, divided by the number of people who were exposed it
The denominator of an incidence rate represents what?
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
Mortality
the term used for the number of people who died within a population and refers to the incidence of death or the number of deaths in a population.
Examples of Point and Period Prevalence and Cumulative Incidence in Interview Studies of Asthma
"Do you currently have asthma?" Point prevalence "Have you had asthma during the last [ n ] years?" Period prevalence "Have you ever had asthma?" Cumulative incidence
How is person-years calculated?
# of participants x years of follow-up per participant
External validity
- "Generalizability" - Degree to which the results of the study may apply, be relevant, or be generalized to populations or groups that did not participate in the study
Ways to express prognosis
- Case fatality - 5-yr survival Observed survival rate
Classification system for cases & disease outcomes
- Confirmed - Probable - Suspected
General reasons for censoring
- Death ( when this is not the endpoint of interest) - withdrawal/drop-out - Administrative censoring (end of study)
Why do we need to express prognosis in quantitative terms?
- Describe the severity of diseased so that the clinical & public health authorities can be established - Answer patient's question about prognosis - Compare effectiveness of currently available therapies or other interventions - Est a baseline for evaluation new treatments
Characteristics Important for Case Definitions
- Onset (sudden, prolonged, etc.) - Duration (acute, chronic, etc) - Recurrence (single occurrence, recurrent, etc.)
Internal validity
- The degree to which a study is free from bias or systematic error. - The soundness of a study design and analysis in answering the question that it posed for study participants. - A prerequisite for external study
Barriers to internal validity
- Vague definition of the target population. - Inability to define the source population - Problems with enrolling the source population
Steps in formulating a public health research question
1.) Define population at risk, exposure of interest and outcome. 2.)Does "A" cause "B"? 3.) Who should be studied? 4.) How do they relate to those eligible to be studied? 5>) How do they relate to those at risk for outcome?
Two methods for open cohort
1.) Life-table method for discrete time periods 2.) Kaplan-Meier method for known event times
How can YPLL assist in functions of public health?
1.) establishing research and resource priorities 2.) surveillance of temporal trends in premature mortality 3.) evaluating the effectiveness of program interventions.
Two steps involved in YPLL
1.) for each cause, each deceased person's age at death is subtracted from a predetermined age at death. (e.g. in the US this is 75. If a person dies at 1, YPLL is 74 (75-1)) 2.) the "years of potential life lost" for each individual are then added together to yield the total YPLL for the specific cause of death.
Two types of incidence measure denominators
1.) people at risk who are observed throughout a defined time period; 2.) when all people are not observed for the full time period, person-time (or units of time when each person is observed).
What 2 factors is the positive predictive value affected by?
1.) the prevalence of the disease in the population tested and, when the disease is infrequent 2.) the specificity of the test being used.
When is mortality rate a good reflection of the incidence rate?
1.) when the case-fatality rate is high (as in untreated rabies), 2.) second, when the duration of disease (survival) is short. Under these conditions, mortality is a good measure of incidence, and thus a measure of the risk of disease. For example, cancer of the pancreas is a highly lethal disease: death generally occurs within a few months of diagnosis, and long-term survival is rare. Thus, unfortunately, mortality from pancreatic cancer is a good surrogate for incidence of the disease.
Age-adjusted rates
A "standard" population distribution is used to adjust death and hospitalization rates. The age-adjusted rates are rates that would have existed if the population under study had the same age distribution as the "standard" population. Therefore, they are summary measures adjusted for differences in age distributions.
Incidence density
A denominator consists of the sum of the units of time that each individual was at risk and was observed. This is used when different individuals are observed for different periods of time.
Source population
A portion of he target population that can be enumerated for the study. It is the subset of the target population
Unimodal curve
A type of distribution with a single peak
Biimodal curve
A type of distribution, in which there are two peaks,
What is the single most important predictor of mortality?
Age
Population definition in epidemiology
Any sizable aggregate of people who satisfy a particular set of membership criteria. Three main characteristics: Person, place and time.
Why should we be concerned about the relationship between predictive value and disease prevalence?
As we have seen, the higher the prevalence, the higher the predictive value. Therefore, a screening program is most productive and efficient if it is directed to a high-risk target population. Therefore, a screening program is most productive and efficient if it is directed to a high-risk target population.
Censored individuals
Censored if... - they did not experience the event of interest during the follow-up up to a specific time and - the occurrence of the event after a specific time is uncertain because they are no longer under follow-up.
Why does specificity have a greater effect than sensitivity on predictive value?
If dealing with infrequent diseases, most of the population falls to the right of the vertical line. Consequently, any change to the right of the vertical line affects a greater number of people than would a comparable change to the left of the line. Thus, a change in specificity has a greater effect on predictive value than does a comparable change in sensitivity. If we were dealing with a high-prevalence disease, the situation would be different.
When Target=Source=Study populations can be the same
If it is feasible to enumerate and study each individual in the target. Example: DB of all people >= 65 yrs of age and enrolled in Medicare is possible because Medicare has that data.
Target population
Group of individuals about whom inferences are to be made. It is sometimes not possible/feasible to enumerate or count the entire target population.
Period prevalence
How many people have had the disease at any point during a certain time period? The time period referred to may be arbitrarily selected, such as a month, a single calendar year, or a 5-year period.
How can occurrence of disease be measured?
It can be measured using rates or proportions. ;
How to calculate the negative predictive value
It is calculated by dividing the number of true negatives by all those who tested negative (true negatives + false negatives). DIVIDED BY total negative tests
How do you minimize false negatives?
Maximize SENSITIVITY --> simultaneous testing
How do you minimize false positives?
Maximize SPECIFICITY --> sequential testing
Dynamic population
Membership is transient and defined by being in or out of a "state", in a certain place, during a certain period of time. i.e., Residents of Philly in 2017
How do you calculate the net sensitivity of sequential testing?
Net sensitivity= sens1 * sens2
How do you calculate the net specificity of sequential testing?
Net specificity= spec1 + spec2- spec1* spec 2
Proportionate mortality
Number of deaths within a population due to a specific disease or cause divided by the total number of deaths in the population during a time period such as a year.
Case Fatality (CF%)
Number of individuals dying during a specified period of time after a disease onset of diagnosis DIVIDED BY Number of individuals with the specified disease TIMES 100 (CF is a proportion)
Rates Vs proportions in disease occurrence
Rates tell us how fast the disease is occurring in a population; proportions tell us what fraction of the population is affected. Examples of rates: incidence rate, mortality rate Examples of proportions: cumulative incidence, case fatality
Steps to identifying newly detected cases of a disease.
Step 1: Screening for prevalent cases at baseline. Step 2: Follow-up and rescreening at 1 year to identify cases that developed during the year.
Types of populations in Epi
Target, source and study
Study population
The group of individuals selected from the source population who participate and are observed in the study. It is the subset of both the source and target populations.
When calculating incidence, what is the numerator?
The number of NEW cases of a disease
Incidence
The number of NEW cases of a disease occurring in the population during a specified period of time.
Incidence rate per 1,000 persons
The number of NEW cases of a disease occurring in the population during a specified period of time. DIVIDED BY The Number of persons who are at risk of developing the disease during that period of time. TIMES 1,000
Prevalence
The number of affected persons present in the population at a specific time divided by the number of persons in the population at that time. (i.e.,what proportion of the population is affected by the disease at that time.) Prevalence is NOT a measure of risk.
Prevalence per 1,000 persons
The number of cases of a disease occurring in the population during a specified period of time. DIVIDED BY Number of persons in the population at the specified time. TIMES 1,000
Cohort effect
The particular impact of a group bonded by time or common life experience
Predictive value
The probability of disease given the results of the test. There are 2 types : Positive predictive value (PPV) & Negative predictive value (NPV)
Risk
The probability that an event will occur during a specified period of time.
Net sensitivity for sequential testing
The proportion of those with the disease who test positive on BOTH test 1 and test 2. How to calculate: Test positive on both test (numerator) DIVIDED BY Total of people w/ the disease (denominator.)
Net specificity
The proportion of those without the disease who test negative on EITHER test 1 and test 2. How to calculate: Negative on test 1 + Negative on test 2 (numerator) DIVIDED BY Total of people WITHOUT the disease (denominator.)
Epidemiology
The study of how disease is distributed in the populations and what factors influence or determine this distribution.
Standardized mortality ratio
a ratio between the observed number of deaths in an study population and the number of deaths would be expected, based on the age- and sex-specific rates in a standard population and the age and sex distribution of the study population.
Specific rate
a restriction is placed on a rate. i.e., if using age, then is is an age-specific rate, using age...age-specific, etc
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. the goal of direct adjustment is to compare rates in at least two different populations when we wish to eliminate the possible effect of a given factor, such as age, on the rates we are comparing.
Closed cohort VS open cohort
Closed proportion - following the same people for the same time period. Open cohort - people may enter and exit at different times
Validity of a test screening
Defined as its ability to distinguish between who has a disease and who does not.
How to calculate the positive predictive value
Divide the number of true positives by the total number who tested positive (true positives + false positives) DIVIDED BY Total positive tests
Proportion
Division of 2 numbers, the numerator is a subset of the denominator. Proportions in epidemiology: who proportion of the population is affected? (Number of people affected / total population)
Rate
Division of 2 numbers, times is in the denominator. A rate involves specification of: - A numerator - A denominator - Time (implicit or explicit) Rates in epidemiology: how fast is a disease occurring? (Number of events/population-time)
Ratio
Division of 2 unrelated numbers
Person-time
Expressed in terms of person-months or person-years of observation. i.e., One person at risk who is observed for one year = one person-year.
Fixed population
Fixed membership is permanent and defined by a certain event, in a certain place, during a certain time period. i.e., 911 bombings
What is the difference between case-fatality and a mortality rate?
In a 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 case-fatality, however, the denominator is limited to those who already have the disease. Thus, case-fatality is a measure of the severity of the disease. It can also be used to measure any benefits of a new therapy: as therapy improves, case-fatality would be expected to decline. You will note that case-fatality is not a rate but a percentage (of those with the disease).
Sequential (Two-stage) Testing
In sequential or two-stage screening, a less expensive, less invasive, or less uncomfortable test is generally performed first, and those who screen positive are recalled for further testing with a more expensive, more invasive, or more uncomfortable test, which may have greater sensitivity and specificity. It is hoped that bringing back for further testing only those who screen positive will reduce the problem of false positives.
Increases/decrease in net sensitivity vs net specificity
In sequential testing - net sensitivity DECREASES and net specificity INCREASES In simultaneous testing - net sensitivity INCREASES and net specificity DECREASES
Net gains and losses in both sensitivity and specificity - how do they different in sequential testing?
In sequential testing, when we retest those who tested positive on the first test, there is a loss in net sensitivity and a gain in net specificity. In simultaneous testing, because an individual who tests positive on any one or multiple tests is considered positive, there is a gain in net sensitivity. However, to be considered negative, a person would have to test negative on all the tests performed. As a result, there is a loss in net specificity. In summary, as we have seen previously, when two sequential tests are used and those who test positive by the first test are brought in for the second test, there is a net loss in sensitivity, but a net gain in specificity, compared with either test alone. However, when two simultaneous tests are used, there is a net gain in sensitivity and a net loss in specificity, compared with either test alone.
Incidence rate if people at risk are observed for different lengths of time
Incidence rate per 1,000 persons observed person-years at risk = The number of NEW cases of a disease occurring in the population during a specified period of time. DIVIDED BY Observed Number of person-years at risk of developing the disease during that period of time. TIMES 1,000
Indirect age adjustment
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: Do people who work in a certain industry, such as mining or construction, have a higher mortality than people of the same age in the general population? Is an additional risk associated with that occupation?
What is the relationship between incidence and prevalence?
Prevalence - incidence X duration of disease
What is the difference between incidence and prevalence ?
Prevalence can be viewed as a snapshot or a slice through the population at a point in time at which we determine who has the disease and who does not. But in so doing, we are not determining when the disease developed. (When we survey a community to estimate the prevalence of a disease, we generally do not take into account the duration of the disease. ) Incidence includes only new cases or events and a specified time period during which those events occurred.
Point prevalence
Prevalence of the disease at a certain point in time—this is the use of the term prevalence that we have just discussed.
What is the relationship between Incidence and Prevalence
Prevalence=Incidence×Duration of Disease
Net specificity for simultaneous testing
Proportion of those WITHOUT the disease who test negative on BOTH test 1 AND test 2. How to calculate: Test Negative on BOTH test (ONLY interested in the overlap!! - multiply by the specificity)(numerator) DIVIDED BY Total WITHOUT the disease (denominator)
Net sensitivity for simultaneous testing
Proportion of those with the disease who test positive on EITHER test 1 OR test 2 How to calculate: Test Positive on EITHER Test (DO NOT COUNT the overlap - multiply by the sensitivity) (numerator) DIVIDED BY Total with the disease (denominator)
Disability-Adjusted Life Year (DALY),
The years of life lost to premature death and years lived with a disability of specified severity and duration. Thus, a DALY is 1 lost year of healthy life. The sum of these DALYs across the population, or the burden of disease, can be thought of as a measurement of the gap between current health status and an ideal health situation where the entire population lives to an advanced age, free of disease and disability.
How can you calculate an observed survival rate?
Using... - person-years - life-tables - Kaplan-Meier method
Case-fatality
What percentage of people who have a certain disease die within a certain time after their disease was diagnosed?
Negative predictive value (NPV)
What proportion of patients who test negative do not have the disease in question?
Positive predictive value (PPV)
What proportion of patients who test positive actually have the disease in question?
Prognosis
a forecast of the likely course of a disease or ailment.
Years of Potential Life Lost
a measure of premature mortality, or early death. YPLL recognizes that death occurring in the same person at a younger age clearly involves a greater loss of future productive years than death occurring at an older age.
