PBH 354 Exam #2

what are the two types of epidemiology?

descriptive and analytic/scientific

purpose of descriptive studies

1. to monitor the public’s health

2. to evaluate the success of intervention programs

3. to generate hypotheses about causes of disease

types of descriptive studies

• case reports and series

• cross-sectional study

• ecological study

what do descriptive studies do?

identify and count cases of disease in populations according to person, place, and time, and conduct simple studies

purpose of analytic/scientific studies

1. to evaluate a hypothesis about the causes of disease

2. to evaluate the success of intervention programs

what do analytic/scientific studies do?

compare group and systematically determine: is there an association?

types of analytic/scientific studies

clinical trial, experimental study, case-control study, and cohort study

what is the goal of epidemiological research?

conduct an epidemiological study to determine the relationship between an exposure of interest and a disease/outcome of interest with validity and precision using a minimum amount of resources

components of a study

1. population

2. exposure

3. outcome

4. potential confounders

5. analysis

6. communication of findings

what is a source population?

the population you are interested in knowing more about

what is a study population?

the population you enroll in your study to represent the source population

what does generalizability mean?

the extent to which the results from a study can be generalized (or extendded) to people who did not participate in the study

case definition

description of the event you are interested in studying; can be a disease, defect, injury, event, state

what is the goal when creating a case definition?

minimize errors

exposure

Determinant of interest upon which an outcome depends; can be constitutional, environmental, or behavioral

examples of constitutional exposures

genetic polymorphism, female sex, short stature

examples of environmental exposures

fluoridated water, air pollution, passive cigarette smoke

examples of behavioral exposures

cigarette smoker, illicit drug user, fitness enthusiast, high fiber diet

goal when determining exposures

minimize errors

cofounders

extraneous determinant of an outcome; can be constitutional, environmental, or behavioral

may lead to distortion of true association between exposure and disease

goal when determining potential confounders

to identify and control them in some way

analysis

examination of your study data; estimation of measures of disease frequency and association

communication of findings

tell the appropriate persons or community what you found even if you found no association

types of experimental analytical scientific studies

• clinical trial

• experimental study

types of observational scientific studies

• cohort study

• case-control study

experimental study

study preventions and treatments for diseases

randomization normally assigns groups

key feature of an experimental study

Investigator assigns individuals to two or more groups that either do or do not receive a preventative or therapeutic agent and follows the groups for incidence of outcomes

setting of experimental study

• ethical and feasible

• small affect expected

• money is available

incidence of outcome for experimental studies

disease occurence, symptom improvement, survival, side affects r

cohort study purpose

studies causes, preventions, and treatments for diseases

key feature of cohort studies

investigator selects subjects according to their exposure levels and follows them for disease occurrence

setting of cohort study

• trial not ethical, feasible, or too expensive

• moderate or large effect expected

• little known about exposure and so can evaluate many effects of an exposure

• exposure is rare

how are groups selected in cohort studies

investigator slects exposed and comparison groups e

retrospective

examines past

prospective

examines future

case-control study purpose

studies cause, preventions, and treatments for diseases

key feature of case-control study

method of sampling a population where subjects are identified as having the disease (cases) or are from the source population that gave rise to the cases (controls) and exposure distribution is compared

setting of case-control study

• trial not ethical, feasible, or too expensive

• moderate or large effect expected

• little known about disease and so can evaluate many exposures

• disease is rare

process of experimental studies

1. research questions for experimental studies

2. types of experimental studies

3. study population

4. consent, enrollment, exposure assignment

5. minimizing bias

6. analysis

equipoise

a state in which investigators truly do not know whether one treatment is better than another, but truly believe withholding treatment, from study participants will not harm them

when is an experimental study ethical

when equipoise is present

types of trials

prevention and therapeutic

prevention trial

agent given to healthy or high-risk individuals to prevent disease occurrence

therapeutic trial

agent given to diseased individuals to treat or cure disease

units of allocation

individual trial and community trial

individual trial

• treatment allocated to individuals

• ex: study effectiveness of treatment with two drugs versus three drugs among adults infected with HIV

community trial

• treatment allocated to entire community

• ex: study impact of drinking water fluoridation on frequency of dental caries in one community vs no fluoridation in similar community

population hierarchy

reference population —> experimental source population (potential participants) —> study population —>informed consent allocation —> treatment and comparison group

sample size is necessary because…

to determine a true difference between treatment and comparison groups, enrollment of an adequate number of study subjects is necessary

statistical power

ability to demonstrate an association

80% or greater = good power

consent process, enrollment, exposure assignment

1. IRB must approve the study protocol

2. The investigator submits a detailed protocol

3. must demonstrate informed consent, privacy and confidentiality, minimization of risks, special protections of vulnerable subjects, and fairness of selection study subject

4. PARTICIPANTS MUST GIVE INFORMED CONSENT

informed consent

clear expectation of research goals and methods, risks and benefits of participating, assurance of confidentiality, right to withdraw, etc.

randomization

each study participant has the same probability of receiving treatment

how does randomization affect confounding variables

minimizes affect

when does randomization work best

• The study is large enough

• treatment assignment is not influenced by investigator

blinding/masking

method of ensuring that participants and/or study investigators have no knowledge of whether a study participant has been assigned to the treatment or comparison group

single blind

study participant does not whether he or she is receiving treatment or no treatment

double blind

neither the study participant nor the study investigator administrating the treatment knows who is receiving treatment or no treatment

triple blind

neither the study partipant, not the study investigator, nor the investigator monitoring the effects of the treatment knows who is receiving treatment or no treatment

ways to minimize bias

• placebo

• compliance

noncompliance makes the study….

more alike

non compliance tends to bias toward the “null'“

cohort study

a study in which people who are free of disease and that differ according to exposure are followed over time for incidence of disease

“natural experiment”

cons of prospective cohort study

• more expensive, time consuming

• not efficient for disease with long laten periods

pros for prospective cohort study

• better exposure and cofounder data

• less vulnerable to bias

pros for retrospective cohort study

• cheaper, faster

• efficient with disease with long latent period

cons for retrospective cohort study

• exposure and confounder data may be inadequate

• more vulnerable to bias

ambidirectional cohort study

contains elements of both prospective and retrospective types of studies

types of cohorts

general and special

general cohorts

investigate exposures that are common in the general population

study multiple exposures on one or more outcomes

special cohorts

investigate unusual and rare exposures

researchers can study effects of single exposure on one or more outcomes

Framingham Heart Study

• objective: identify characteristics that contribute to CVD by following healthy individuals

• 5,209 men and women between 30-62 years old recruited

• 1948 first generation

• 1971 second generation

• 2002 third generation

Air Force Ranch Hand Study

• 11 million gallons of herbicides sprayed from 1962 to 1971 in Vietnam

• airmen and the local population were exposed

counterfactual ideal

ideal comparison group would be the exact same people who are in the exposed group had they not been exposed —> epidemiologist select different sets of people who are as similar as possible

types of comparison groups

• internal

• comparison cohort

• general population i

internal

within one type of population

ex: nurses

comparison cohort

two different group

ex: landscapers and construction workers

general population

a subgroup compared to the general population

healthy worker effect

rates of morbidity and mortality among the working population are lower than those of the general population —> leads to underestimation of risk when comparing a worker population to the general population

what type of comparison group leads to the “healthy worker effect”

general population comparison

high levels of loss to follow up can….

result in a bias

how much loss to follow up is okay

less than 80%

strengths of cohort studies

• clear temporal sequence between E and D, especially if prospective

• efficient for rare exposures

• goof information on exposures, confounders, especially if prospective

• can study effects of an exposure on multiple outcomes

limitations of cohort studies

• may need large numbers of subjects to be followed for long period of time

• can be expensive and time-consuming

• not good for rare diseases or those with long latency, if prospective

• LTF has potential to undermine validity

what is a solution to cohort studies?

case-control studies

case-control studies

• a study in which cases of disease are identified, and then a sample of the population that produced the cases is identified

• exposures are determined and compared for individuals in each group

process of case-control studies

1. define the study hypothesis

2. define the source population

3. identify the cases from the source population

4. identify valid controls from the same source population

5. ascertain exposure for cases and controls

6. calculate a measure of association between exposure and outcome

requirements for control selection

1. controls must come from the same source population as the cases

2. controls must be selected independently of exposure

would criterion

if the controls had experienced the outcome, would they have been identified as cases in your study

odds ratio

relative measure of association

NO absolute measures of association can be calculated

risk of illness among exposed

developed disease/ exposed persons

odds of illness among exposed

developed disease/remaining healthy

odds ratio formula

(a/c)/(b/d)

interpretation of odds ratio calculation

compared to unexposed subject, exposed subjects have a ___ times the odds of getting the disease

no association for odds ratio

OR = 1

increased odds

OR>1

decreased odds

OR<1