US exam 3 epidemiology Flashcards
Conducting studies
Studies are conducted in an attempt to discover associations between an exposure or risk factor and a health outcome
O1: Epidemiology Study Types
epidemiology study types:
observational: descriptive or analytic
experimental
Descriptive and Analytic Epidemiology
Descriptive epidemiology
- When was the population affected?
- Where was the population affected?
- Who was affected?
Analytic epidemiology
- How was the population affected?
- Why was the population affected?
Distribution
Who: age, sex, race and economic status
When: trends of disease frequency over time (is it increasing?, decreasing?, remaining stable?)
When: tracking an outbreak. Use epidemic curves to plot the number of cases over a period of time.
Where: compares disease frequency in different counties, states or countries or other geographic areas. Can also compare urban vs. rural areas
Distribution
The information gathered on the distribution of disease gives clues about the determinants of disease.
Determinants can be any underlying social, economic, cultural or environmental factors that are responsible for health and disease.
O1: Study Designs
Three study designs are commonly used in epidemiology (Observational)
Cross-sectional
Cohort
Case-control
Experimental studies to test efficacy of medications
Randomized Controlled Studies are used
Will go into further detail next semester in Drug Lit
Determinants
When you have clues to the determinants of disease you can generate a hypothesis
The hypothesis can be tested by formal/systematic epidemiologic studies
These can confirm or disprove the hypothesis
Study Design-Cross Sectional
what does it analyze
how are subjects selected
what does it provide
Analyzes data of variables collected at one given point in time across asamplepopulation or a pre-defined subset.
Subjects are selected because they are members of a certain population at a certain period of time
Provides a “snapshot” of exposures and outcomes
Study Design-Cohort Study
what does it study
what does it start with
what does it provide the best of
what are they in nature and how long does it take to complete it
A study of a group of people, or cohort, followed over time to see how some disease or diseases develop
Study typically begins with healthy subjects and are asked about their exposures
These studies provide the best information about the causation of disease and the most direct measurement of risk of developing a disease
Prospective or retrospective in nature can take years to complete
O2: Cohort Studies
what are the advantages
- how many outcomes can be studied
- what does it provide
- - what is not likely
- how can you directly calculate
what are the disadvantages
- how much money
- how long does it take to complete
- what is it inefficient for
- what may be missing
- what can there be loss in
Advantages
Can study multiple outcomes
Study uncommon & multiple exposures
Provides clearer sequence of events
Selection bias is not likely
Directly calculate disease incidence
Disadvantages
Expensive & time consuming
May take years to complete study
Inefficient for rare outcomes or disease with long latent periods
Data on some confounding variable may be missing
Source of error
Loss to follow-up
Study Design- Case Control
what does it compare
who are the case patients
who are the control patients
how do epidemiologists work
what other studies can it be tested within
A study that compares individuals affected by a disease with a comparable group of persons who do not have the disease to determine possible causes or associations
Those with the disease or condition are case-patients
Those without the disease or condition are control-patients
Epidemiologists work backward from the illness or health
condition (retrospective in nature) to determine any associations - so how did you get the disease
Can be nested within a cohort study-more on this in Drug Lit
Study Design- Case-Control
Cases and controls are compared for the presence or absence of one or more specific exposures or risk factors
Takes a shorter amount of time to complete
All subjects are matched as much as possible (age, race, gender, other factors relative to the disease)
O2: Case Control Studies
advantages
disadvantages
Advantages
Can study multiple exposures
They are efficient for rare diseases or diseases with a long latency period between exposure and disease manifestation
Faster and easier to conduct than other studies
Less expensive
Disadvantages
Bias can be a source of error
Recall Bias
Information Bias
Reporting bias
Selection bias
They are inefficient for rare exposures
Experimental Study
what is this study used for
what are the two groups
how can the two groups be
how are subjects placed into groups
Randomized Controlled Trials (RCT)- Will be covered in detail next semester!
These types of studies are used to test new drugs for safety and efficacy before they are approved for marketing
Start with two groups:
Experimental group-will get the new treatment
Control group-may get placebo
Can be blinded or double blinded
Subjects are randomized into groups
Have an intervention occur (subjects given counseling and drug or no counseling with placebo)
O2: Randomized Controlled Studies
Advantages
Convincing
Can control for confounders (known and unknown)
Disadvantages
Very expensive- millions of $$
Artificial environment
Ethical issues
Difficult to conduct logistically
O3: Measurements used in epidemiology
Frequency:
Rate is the number of cases but this number must be relative to the size of the of the population being studied.
Population at risk (PAR): This number should only include people who are potentially susceptible to the disease being studied.
Measurements used in epidemiology: incidence
- what does it measure
- what does it express
- what must always be included
Incidence is the rate of new cases of a disease in a defined population over a defined period of time
Measures the probability that a healthy person in that PAR will develop that disease during that specific time
Incidence-Expresses the risk of becoming ill.
Must always include a unit of time- such as cases per 10n per day, week, month or year.
prevalence
what is prevalence
how fast to rates change and how useful is it in epidemiology
what does it estimate
how is it expressed
what is mortality rate
Prevalence is the total number of cases of a disease existing in a defined population at a specific time
Rates change slowly-not useful for epidemiologic studies
Useful in assessing social impact of disease-affects planning
Prevalence- Estimates the probability of the population
being ill at the period of time being studied
Prevalence is often expressed as cases per 100 (percentage) or per 1000 population.
Mortality rate is the incidence of death per unit of time (usually per year) in a population, can look at all deaths or a specific cause of death
O4: Measurements of Association
Epidemiology identifies associations between exposures and outcomes
Salt intake → Hypertension (direct association)
Salt in take → Hypertension → Coronary artery disease (in-direct association)
Types of measurements used in epidemiologic studies
Relative Risk
Odds Ratio
Measurements of Association: Relative Risk (RR)
If an association exist, how strong is it?
What is the ratio of the risk of disease in exposed individuals to the risk of disease in unexposed individuals?
RR = 1.0= no association between exposure and the disease
RR > 1.0= shows a positive association (increased risk from exposure of getting the disease)
RR < 1.0 = Shows a negative association (decreased risk from exposure of getting the disease)- sometimes called a “protective effect”
Typically used in COHORT studies
Measurement of Association: Odds ratio (OR)
what does it ask
what is it used in
what can you not directly calculate
what does each mean
OR = 1
OR >1
OR < 1 negative association
What are the odds that the disease will develop in an exposed person? Association of an exposure and a disease
Used in a case-control studies
Can not directly calculate the risk since subjects already started with the disease- it is an approximation of the relative risk
OR = 1 no association, OR >1 positive association, OR < 1 negative association
Measurement of Association: 95% Confidence Interval
The 95% confidence interval (CI) is used to estimate the precision of the OR (also used in RR).
A large CI indicates a low level of precision of the OR, whereas a small CI indicates a higher precision of the OR.
In practice, the 95% CI is often used as a proxy for the presence of statistical significance if it does not overlap the null value (e.g. OR=1).
Will look like: RR=3 (95% CI 1.67-4.2); p<0.5 it is a range of numbers
a range of values so defined that there is a specified probability that the value of a parameter lies within it.
p-value: if we did the experiment 100 times, 95 of the tests will be the same and 5 of them will be different
we are 95% confident that the true value lies between that interval especially if it a small/narrow interval
O5: Sources of Error
Confounding variables:
Is a factor or explanation that may affect a result or conclusion.
Sources of Errors
what is bias
what is selection bias
what study is this a problem for
what si recall bias
Bias- is a systematic error in the design, conduct or analysis of a study that results in a mistaken estimate of an exposure’s effect on the risk of disease.
Selection bias: there is an error in choosing the individuals or groups to take part in a study
This is often a problem in case-control studies
Recall Bias: An error caused by differences in the accuracy or completeness of the recollections retrieved (“recalled”) by study participants regarding events or experiences from the past.
Is an issue in case-control studies
O7: Proving Cause and Effect
Epidemiological studies usually speak of risk factors than causes.
To make the results of a study stronger it is important to show a cause and effect relationship
Elements of Cause and Effect
A study with a large number of subjects is more likely to yield a valid result
The Framingham Heart Study has thousands of participants
This makes the results much more valid than a study of only 30 subjects
