Study biases and Hypothesis testing Flashcards
What is selection bias?
Nonrandom assignment to participate in a study group.
Most commonly a sampling bias.
What kind of bias may be present in a study with many patients who are “lost to follow up”?
A selection bias - the disease may have early mortality, leading to a loss of certain participants.
What kind of bias may be present in a study with healy workers and volunteers?
Selection bias - Study populations are healthier than general population.
How can selection bias be reduced?
Randomization
Ensure the choice of the right comparison/reference group.
What is recall bias?
Awareness of disorder alters recall by subjects - common in retrospective studies.
Patients with disease recall exposure after learning about similar cases.
How can you reduce recall bias?
Shorten the time from exposure to follow-up.
What is measurement bias?
Information gathered in a way that distorts it.
Example: Hawthorne effect - groups who know they’re being studied behave differently.
What is procedure bias?
Subjects in different groups are not treated the same.
What is observer-expectancy bias?
Researcher’s belief in the efficacy of the treatment changes the outcome of that treatment.
Example: If observer expects treatment group to show signs of recovery, more likely to document positive outcomes.
How can measurement, procedure, and observer-expectancy bias be reduced?
Use of placebo control groups with blinding to reduce influence of participants and researchers on experimental procedures and interpretation of outcomes.
What is a confounding bias?
When a factor is related to both the exposure and outcome but not on the causal pathway.
Factor distorts or confuses effect of exposure on the outcome.
Example: Pulmonary disease is more common in coal miners than general population. However, coal miners are also more likely to smoke than the general population.
How can confounding bias be reduced?
Multiple/repeated studies.
Crossover studies (subjects act as own controls)
Matching (patients with similar characteristics in both treatment and control groups)
What is lead-time bias?
Early detection is confused with longer survival, seen with improved screening techniques.
Early detection makes it seem as though survival has increased, but the natural history of the disease is unchanged.
How can lead-time bias be reduced?
Measure “back-end” survival (adjust survival according to severity of disease at time of diagnosis).
What are the measures of central tendency?
Mean, median, and mode.
How is the shape of a normal distribution described?
In a normal distribution, how many of the values are one standard deviation away from the mean?
How about two?
Three?
Gaussian/bell shaped.
1SD from the mean: 68%
2SD: 95%
3SD: 99.7%
How is a positively skewed distribution shaped?
What will the relative size of the mean, median, and mode be?
Asymmetry with a longer tail on the right.
Typically Mean > median > mode
How is a negatively skewed distribution shaped?
What will the relative size of the mean, median, and mode be?
Asymmetry with a longer tail on the left.
Typically mean < median < mode
