Epidemiology
blank.
L1_Overview_of_Epidemiology_18.pptxOverview of Epidemiology
Make sure you can see the phrase “I love epi” in the notes pages below (the white space)
1
I love epi
EPIDEMIOLOGY
Greek word
Epi – on or upon
Demos – people
Logos – doctrine (the study of)
The study of who gets sick and why
2
Epidemiology, nicknamed the “Queen Science,” is the fundamental science of public health. However, it status as such was slow in coming: The first epidemiological society in England was formed in 1850, but the first text book wasn’t published until the 1960s. Epi is now a cornerstone of many sciences, and is an integrated discipline that includes medicine, statistics, biology, demography, computer sciences, etc.
Epi is based on two primary assumptions: 1) Human disease does not occur at random; and, 2) human disease has causal and preventative factors that can be identified through systematic identification of different populations or subgroups within a population in different places or at different times. Thus, epi is the basic study of the process of disease causation (etiology). Epidemiologists measure the rates of disease occurrences in populations to try to answer the question "Why did some people become ill with this disease while others did not?" They attempt to identify risk factors and contributing causes for specific diseases, so that other public health practitioners can intervene in order to prevent new cases of the disease from occurring.
Definition Of Epidemiology
The science or study of the distribution and determinants of disease in a human population
Epidemiology is the study of the determinants and distribution of diseases in human populations. It’s primary goal is disease prevention.
Key terms:
Determinants: Contributing causes (emphasis on “contributing”)
Note: Causes and diseases do not correspond 1:1; that is, just because you have the determinant (for example, you smoke), doesn’t mean you have – or will get – the disease (lung cancer).
Distribution: Relative commonness of disease in different segments of the population. Concerned with:
Who is getting the disease
Where is the disease occurring
When is the disease occurring
Note: Knowledge of the distribution is essential to describe patterns of disease as well as to formulate hypotheses concerning possible causal or preventative factors
Disease: deviation from physical, mental, emotional health
Population: large group, often geographically defined, but can be defined by a characteristic such as gender or ethnicity.
3
Objectives of Epidemiology
Determining extent of disease in a population
Identifying disease patterns and trends
Identifying the causes of disease
Evaluating effectiveness of measures that prevent and treat disease
Epi has four broad objectives:
• Determining the extent of disease in a population.
• Identifying patterns and trends in disease occurrence
• Identifying the causes of disease
• Evaluating the effectiveness of measures that prevent and treat disease
The first two objectives are part of “descriptive epi” which simply describes what exists in a population. The epidemiologist collects information to characterize and summarize the health event or problem. Descriptive epi is useful in demonstrating trends and generating hypotheses about disease causation. We will spend several lessons on descriptive epi.
The third, and sometimes the fourth, objectives are part of “analytical epi” where epidemiologists make inferences based on the data
they collect. These inferences are the bases for hypotheses, which are tested in an attempt to discover the relationship between an exposure or risk factor and a health outcome. We will spend several lessons on analytical epi.
The fourth objective is sometimes part of “clinical” or “experimental” epi. I’ll go over what this is in a later lesson, but we will not cover it in any depth in this course.
4
Epi & Public Health
Surveillance
Screening
Outbreak investigation
Assessing causation
In public health, the aforementioned objectives are carried out through these four core epidemiological processes:
Surveillance: The regular collection, meaningful analysis, and routine distribution of relevant data that provides opportunities for public health action to prevent and control disease.
Screening: The identification of an unrecognized disease or defect by the application of tests, examinations, or other procedures. Screening tests are used to determine D+ and D- in people who are apparently well
Outbreak investigation: A multi-step process for determining the dynamics of a disease outbreak and implementing control and prevention measures.
Assessing causation: Determining the relationship between disease and exposure to risk factors.
In this course, we will briefly cover screening and surveillance. Outbreak investigations are covered in PH581.
5
Critical Measurements of Epi
1. Exposure (E)
2. Disease (D)
6
In epi, there are two things we are primarily interested in measuring:
1. Exposure (E): This can be something in the environment, such as air pollutants or lead paint, or something that a person takes/does, such as medications or smoking. These are the things we would classify as “determinants”
2. Disease (D): An illness, health condition, etc.
So this seems very straight forward, right? Well, it’s not as straightforward as it would seem… The challenge with E & D is in how we define and measure each. There must first be a very clear definition of what constitutes a case (the epi word for someone who has the disease of interest) and what constitutes an exposure (exposed once? Exposed over a certain length of time?). And these definitions must consistently applied by everyone conducting the epi investigations. Without these standard definitions that are consistently applied, we cannot compare results between those with the exposure (E+) and those without (E-), or those with the disease (D+) and those without (D-). We will talk more about this in a later lesson. =)
FYI: When measuring D, case definition may be based on:
1. Signs (objective)
2. Symptoms (subjective)
3. Medical tests
In many studies, epidemiologists accept the physician's diagnosis as "the truth" either because the diagnosis of that disease is unequivocal, or because it is difficult or impossible to re-confirm the diagnosis, either because of passage of time, or, in some cases, passing of person…. The problem accepting the MD’s diagnosis is that not all MDs will make good or consistent diagnoses…
Note: I will sometimes write diagnosis as dx and treatment as tx
Philosophy of Causation
If exposure to E and occurrence of D seem to go together, does E cause D?
7
So the philosophy of causation – the big picture of epi – is this: If exposure to E and occurrence of D seem to go together, does E cause D? For example, if Sue smokes cigarettes and then develops lung cancer, was the lung cancer because of (due to) smoking?
We would like to answer this question because of the public health implications for preventing disease occurrence in other people.
WARNING!
Be careful when attributing causation from epi data…
So, there are times when we may observe that D follows E in many people. But does that mean D will follow E for the next person? Does "causation" mean that D will follow E?
Many scientists believe that causation cannot be proven by epi studies alone because of a) scientific reasoning problems and b) its observational study design. In order to characterize or explain a phenomenon such as causation, we must instead set up a model (or system of a set of rules) to explain the biological phenomenon (people getting sick) that we observe. Otherwise, we may draw very faulty conclusions… let’s take at look at how, shall we?...
More than 98% of convicted felons eat bread.
50% of all children who grow up in bread consuming households score below average on standardized tests.
In the 18th century, when virtually all bread was baked in the home, the average life expectancy was less than 50 years; infant mortality rates were unacceptably high; many women died in childbirth; and, diseases such as typhoid, yellow fever, and influenza ravaged whole nations.
More than 90% of violent crimes are committed within 24 hours of eating bread.
Primitive tribal societies that have no bread exhibit a low incidence of cancer, Alzheimer’s, Parkinson’s disease, and osteoporosis.
Bread has been proven to be addictive. Subjects deprived of bread and given only water to eat begged for bread after as little as two days.
Bread is often a “gateway” food item, leading the user to “harder” items such as butter, jelly, peanut butter, and even cold cuts.
Each of these statements document the relationship between bread (our exposure) and various diseases (crime, illness, etc.) So, E precedes D. But does bread really cause criminal behavior? Low test scores? Of course not! We’ll learn later in the course how to show this, but for now, be cognizant that just because there’s a relationship between E and D, there is not necessarily causation…
8
