HSCI 330 Lecture Notes - Lecture 3: National Health Interview Survey, Cumulative Incidence, Cohort Study
24 May 2018
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Module : Measues of Disease Feue ad
Effet
Measures of Disease Frequency: 4-1 Incidence VS. Prevalence
INCIDENCE
PREVALENCE
❖ Measures new cases of a disease that develop over
a period of time
❖ Describes the input flow of new cases into the pool
❖ Useful for identifying risk factors and assessing
disease etiology
❖ Estimated from clinical trials and cohort studies
(involve follow-up of subjects over time)
❖ Measures existing cases of a disease at a particular
point in time or over a period of time
❖ Viewed as describing a pool of disease in a population
❖ Estimated from cross-sectional studies and case-
control studies
❖ Most useful in the planning of health services (# of
prevalent cases indicates demand for health care)
Mortality used instead of Disease Incidence
❖ Important tool for epidemiologic surveillance
❖ Surveillance programs applied to monitor occurrence of a wide variety of health events (ex. Deaths in large pop.)
❖ Motalit statistis oeiet fo ealuatig etiologi hpotheses, esp. he iidee data ist aailale
❖ Gives reasonable approx. to incidence measures for diseases with low rate of cure//recovery (ex. Lung cancer)
Advantages:
❖ Mortality data are widely collected and virtually complete since registration of deaths is compulsory in most
industrialized countries and few deaths are not reported.
❖ Mortality data are defined using standardized nomenclature. In particular, the International Classification of
Diseases (ICD) is used to promote uniformity in reporting causes of death.
❖ Recording of mortality data is relatively inexpensive
House Guest Example:
A scenario involving houseguests who arrive at 2 per day and stay five days illustrates the fundamental difference
between incidence and prevalence.
❖ A steady state of 10 houseguests illustrates prevalence, which describes the existing count of guests at any point
in time after steady state is reached.
❖ The arrival of 2 guests per day illustrates incidence, which describes how quickly new guests are arriving.
❖ The duration of 5 days is the information needed to link how incidence leads to prevalence
❖ Prevalence is obtained as the product of incidence and duration (P = I x D)
Quiz (Q4.1)
For each of the following scenarios, determine whether it is more closely related to incidence or to prevalence.
1. Number of campers who developed gastroenteritis within a few days after eating potato salad at the dining hall? ____
2. Number of persons who reported having with diabetes as part of the National Health Interview Survey? ______
3. Occurrence of acute myocardial infarction (heart attack) among participants during the first 10 years of follow-up of
the Framingham Study? ____
4. Number of persons who died and whose deaths were attributed to Hurricane Floyd in North Carolina in 1999? _____
5. Number of children who have immunity to measles, either because they had the disease or because they received the
vaccine? ____
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Suppose a surveillance system was able to accurately and completely capture all new occurrences of disease in a
community. Suppose also that a survey was conducted on July 1 that asked every member of that community whether
they currently had that disease. For each of the following conditions, determine whether incidence (per 1,000 persons
per year) or prevalence (per 1,000 persons on July 1) is likely to be higher.
6. Rabies (occurs rarely and has a short duration, e.g., death within one week)? _____
7. Multiple sclerosis (rare occurrence, long duration [many years])? _____
8. Influenza (common but winter-seasonal occurrence, short duration)? _____
9. Poison ivy dermatitis (common spring/summer/fall occurrence, 2-week duration)? _____
10. High blood pressure (not uncommon occurrence, lifelong duration)? _____
4.2 Risk
Risk= describes the probability that some event of interest will occur
Ex. What is the risk that the stock market will crash or that we will be involved in a serious auto collision?
Risk = the probability that an individual with certain characteristics (age, race, sex etc.) will develop or die from a disease
(health status change of interest over a specified follow-up period)
❖ Risk is the probability than an individual will develop or die from a given disease or, more generally, will
experience a health status change over a specified follow-up period.
❖ Risk assumes that the individual does not have the disease at the start of the follow-up and does not die from
any other cause during the follow-up.
❖ Health outoe is a disease defiitio assues that idiidual doest hae disease at the stat of follo up o
die during it)
❖ Risk is a probability → number between 0 and 1 or correspondingly a %
❖ When describing risk, it is necessary to specify a period of follow-up (called RISK PERIOD)
o Ex. Risk that a 45-year-old male will develop prostate cancer → state risk period, 10 years of follow-up
over which we want to predict the risk.
OR
Cumulative Incidence
❖ Cumulative incidence (CI) is a population-based estimate of individual risk
❖ Cumulative incidence is always a proportion
❖ When describing cumulative incidence, it is necessary to give the follow-up period over which the risk is estimated.
❖ The formula for simple cumulative incidence is CI=I/N, where I denotes the number of new cases of disease that
develop over the follow-up period and N denotes the size of the disease-free population at the start of follow-up.
❖ The terms cumulative incidence and risk are used interchangeably in this course, even though technically, they are
different.
CI = Entire population
= estimate of cumulative incidence is based on a sample
Example:
As a simple example, suppose we followed 1000 men age 45 and found that 50 developed prostate cancer
within 10 years of follow-up and that no subject was lost to follow-up or withdrew from the study. Then our
estimate of simple cumulative incidence is 50 over 1000, or 0.05, or, 5 %.
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Shifting to Cohort
“iple uulatie iidee = assues that ohot is fied (no entries into cohort allowed during follow-up
period)
❖ If subjects progressively enter the study at different calendar times, the data can be shifted to reflect the time of
observation since initial entry.
❖ Simple cumulative incidence can be used to estimate risk for a shifted cohort.
❖ After shifting the cohort, we can compute cumulative incidence provided all subjects who remained disease-free
throughout follow-up are followed for the entire length of follow-up.
Problems with Simple Cumulative Incidence
❖ There are problems with assuming a fixed cohort when using the formula for simple cumulative incidence to
estimate risk.
❖ If there is attrition of a fixed cohort, we will not know whether a subject lost during follow-up developed the
disease.
❖ For a dynamic cohort, the denominator in the simple cumulative incidence formula does not reflect the continually
hagig populatio size ™ “iple uulatie iidee does ot allo sujets to e folloed fo diffeet peiods
of time.
1. Size of a fixed cohort is likely to be reduced during follow-up period as a result of deaths or other sources (loss to
follow-up or withdrawal)
2. If population studied is a DYNAMIC POP. rather than a fixed cohort (group of subjects identified at point in time and
followed for a given period for detecting new cases)
3. Subjects may be followed for different periods of time so that a cumulative incidence estimate will not make use of
differing follow-up periods (occurs when subjects lost to follow-up or withdrawn from study)
Study Questions (Q4.4)
The questions below refer to the figure below:
1. What type of cohort is being studied, fixed or dynamic?
2a. Which of these subjects was diagnosed with the disease?
Subject 2 Subject 3 Subject 5 Subject 7
2b. Which of these subjects was lost or withdrawn?
Subject 2 Subject 3 Subject 5 Subject 7
2c. Which of these subjects died with disease? Subject 3
Subject 5 Subject 7 Subject 9
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