W4 Random sampling error, bias, confounding Flashcards
Today’s Class
-validity of studies in health research
-major threads to validity
–bias
—selction
—infromation (happens due to measurement error, last week)
-Confounding
Epidemiology and Causal Inference
-Goals of Epidemiology and health research…
-Building Blocks…->
Results of research…
-identification of causes and preventions for disease
-measures of Disease Frequency, Various Study Designs
Measures of Association
Once you have calculated a measure of association, you need to determine if the observed association is valid and if it is causal
–assessment of validity is a methodological undertaking
–evaluation of the process of ‘causal inference’ is more a philosophical discussion
Research Evidence
Strong evidence is (1…2…)
1) of the lowest possible random sampling error (a statistically significant exposure/outcome association)
2) Based on a good design
-free of selection and information biases
-under minimal influence of confounding
High validity! Of the study and the measures association
Not the validity of the measure of events or exposure
Internal Validity
do the observed results accurately reflect the true association?
Generalizability (External Validity)
- to whom can results be applied?
- requires internal validity
What are key things to note about internal validity and External Validity (generalizability)?
- If a study lacks internal validity, external validity is irrelevant
- We do not compromise internal validity in an effor to achieve external validity (generalizability)
External validity
-will be achieved by a sample that represents the target population
–also by weighting
—more in survey/data collection methods courses
do not confuse with selection bias( healtheir, younger, educated, etc. more likely to volunteer for research)
How do we determine whether our MEASURED ASSOCIATIONS are true (valid)?
Four hallmarks of Health Studies
1) A research question/plausible theory
2) a well thought design to address the research question
3) MEASUREMENT of exposure and outcome
4) Analysis to COMPARE groups (measure association)
-suboptimal design, imperfect measurement, and influence of other factors threat the VALIDITY of health/ epidemiological studies
- also we assumed there is no other factor except exposure and outcome
If we observe an association
-What are the two apparent associations?
First consider(3) before ensuring it is a true association:
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If we observe an association
-What are the two apparent associations?
-Exposure + Disease/Outcome
First consider(3) before ensuring it is a true association:
-due to chance… if chance unlikely then:
-due to bias……If no then:
-due to confounding…..If no then:
-true association… YES
Validity is..
- having fewer errors (errors=measured value- True value)
Sources of error:
–chance (random sampling error)
–bias: systemic error in selection of participants and/or measurements
–Confounding
Threats to Validity
Threats to Validity
Random sampling error
1) Random error in measurement
–information bias
2) Randomization ( a process in experimental studies)
-role of chance and statistics
-sample variation, sample to sample differences
Random Sampling (week 2)
-selection techniques wherein the probability of selecting each sampling unit is known
-following laws of probability
–12 participants out of 36 are selected RANDOMLY
–chance of being in the sample is THE SAME for each of the 36 member of the population equal to 1/36
Random Sampling Error
-variability in sampling due to chance
-statistical analysis cant FIX this, reported p. value only shows how much THE OBSERVED RESULTS may be only DUE TO CHANCE IN RANDOM SAMPLING
-a wide confidence interval suggest a high probability of random sampling error
The best way to MINIMIZE random sampling error is to increase the sample size
What is a P. value?
Threats to Validity (check if makes sense)
1) chance
2) bias
Threats to Accuracy(check if makes sense)
Bias
Bias
-refers to a systematic error in the design or conduct of a study
-when bias occurs in a study the OBSERVED association between the exposure and outcome will be DIFFERENT from the TRUE association
Most biases relate to the STUDY DESIGN AND PROCEDURES and can be classified into categories:
-selection bias
-information bias (due to measurement error)
Types of Bias
(1) Selection Bias
(2) Information Bias
Is it possible to have both types of bias in the same study?
(1) WHO is in the 2x2 table
(2) WHERE in the 2x2 table
It’s possible to have both types of biases in the same study.