9 - Observational Research 1-3** Flashcards
(41 cards)
1
Q
What are observational studies?
A
- Studies of drugs in large groups
- Like pharmacoepidemiology
2
Q
What is the Bradford Hill criteria of causality to validate cause and effect?
A
- Biologically plausible
- Be strong
- Reflect a biological gradient (dose response relationship)
- Be found consistently
- Holds over time (temporal incidence of the disease should reflect prevalence of offending agent)
- Confirmed by experiment
3
Q
What is a case report?
A
- Medical hx of a single patient in “story” form
- Usually b/c some odd, one-off case stands out from typical medical case
4
Q
Advantages of case reports
A
- Fast
- Individual detail
5
Q
Disadvantages of case reports
A
- Not an experiment
- Not controlled
- Weak evidence, bottom of evidence hierarchy
6
Q
What is a case series?
A
Group of case reports
7
Q
What do cross sectional studies determine?
A
Prevalence (proportion of the population w/ a disease/risk factor at one point in time)
8
Q
What is the difference between cross sectional, cohort, and case-control studies?
A
- Cross section = present disease and exposure
- Cohort = present exposure and future disease
- Case control = present disease and past exposure
9
Q
Advantages of cross sectional studies
A
- Cheap, fast
- Collect info on disease and risk factors at the same time
10
Q
Disadvantages of cross sectional studies
A
- Establishes association, not causality
- Recall bias issue (ex: surveys)
- Confounders unequally distributed
- Group sizes are unequal
11
Q
Describe case control studies
A
- Compare an exposure or risk factor between cases/ controls
- Compute an odds ratio, 95% confidence interval, and p-value
12
Q
What is an example of a question posed in a case control study?
A
Were individuals seen at a pediatric oncology clinic at increased odds for renal injury if they were exposed to vancomycin?
13
Q
What is an odds ratio?
A
- An effect estimate
- Measures association between an exposure and outcome, risk factor and outcome, or a predictor and outcome
- Ex: exposure (drug) and outcome (drug response; adverse event)
14
Q
What does an odds ratio of 1 mean?
A
- Exposure doesn’t affect odds of outcome
- Equal odds for both groups
15
Q
What does an odds ratio above 1 mean?
A
- Exposure associated w/ higher odds of outcome
- Ex: OR = 2 means that those w/ vancomycin were at 2x increased odds for renal injury compared to those who didn’t take vancomycin
16
Q
What does an odds ratio less than 1 mean?
A
- Exposure associated w/ lower odds of outcome
- Ex: OR = 0.5 means that those w/ vancomycin were at 50% decreased odds for renal injury compared to those who didn’t take vancomycin
17
Q
What is the purpose of confidence intervals?
A
- Used to estimate the precision of the OR
- Large CI indicates low level of precision of OR
- Small CI indicates high precision
- Unlike p-value, 95% CI doesn’t report a measure’s statistical significance
- Often 95% CI is used as a proxy for statistical significance if it doesn’t overlap OR = 1
18
Q
How does sample size affect CI?
A
Small sample size = wider CI and vice versa
19
Q
How do you calculate odds ratio?
A
- OR = ad/bc
- A = # of px w/ risk factor and disease
- B = # of px w/ risk factor but no disease
- C = # of px w/ no risk factor but disease present
- D = # of px w/ no risk factor and no disease
20
Q
Advantages of case control studies
A
- Good for rare diseases/ outcomes (able to “enrich” for outcome)
- Good for outcomes w/ long latency
- Quicker to do
- No need for follow up
- Smaller sample size
- Cheaper
21
Q
Disadvantages of case control studies
A
- Difficult for rare exposures
- Small sample size
- Sometimes difficult to find appropriate control group
- Can’t estimate risks
- Interviewer bias
- Recall bias
22
Q
Describe cohort studies
A
- Begin w/ sample (“healthy cohort”; subjects w/o the outcome yet)
- Start w/ exposure status, then compare subsequent disease experience in exposed vs. unexposed
23
Q
What is the largest difference between case control and cohort studies?
A
- When the outcome is measured
- Case control = before
- Cohort = after
24
Q
Describe prospective cohort studies
A
- Have a population (large, undefined size)
- Sample from population
- Measure predictor variable
- Measure outcome/ disease in follow up
25
Advantages of prospective cohort studies
- Can study multiple outcomes from a risk factor
- Prospective, so can measure variables completely and accurately
- Fatal diseases are easier to study prospectively than retrospectively
- Very useful when exposure is rare
- Time sequence strengthens inference
- Can provide a strong suggestion of causality
26
Disadvantages of prospective cohort studies
- Expensive and time consuming
- Need large # for a long period of time to have sufficient events to produce meaningful results
- Associations found may be due to confounding factors (ex: alcohol may be a confounder in a study of smoking)
- Loss to follow up a problem
27
Describe retrospective cohort studies
- Design essentially the same as prospective except that baseline measurements, follow up and outcomes have all occurred in the past
- Need adequate data about risk factors and outcomes assembled for other purposes
- Identify a cohort that has been assembled in the past, collect data on predictor variable, and collect outcome/disease data in the past or present
28
Advantages to retrospective cohort studies
- Measurements collected before the outcomes are known; not biased by outcome of interest
- Less costly and time consuming than prospective studies (measurements already taken and follow-up period already occurred)
29
Disadvantages to retrospective cohort studies
- Limited control over the nature and quality of the predictor variables
- Existing data may not include subjects or variables that are important to answering research questions
- Inaccurate or incomplete variable measurement
30
What is a hazard ratio?
- Measures association between an exposure and an outcome
- Ex: exposure (drug) and outcome (drug response; AE)
- Similar to odds ratio but better
- - Represents instantaneous event rate (probability individual would experience an event at a particular time after intervention)
31
What does a hazard ratio of 0.5 mean?
At any particular time, 1/2 as many px in the tx group are experiencing an event compared to the control group
32
What does HR = 1 mean?
At any particular time, event rates are the same in both groups
33
What does HR = 2 mean?
At any particular time, twice as many px in the tx group are experiencing an event compared to the control group
34
Describe nested case control studies
- Start w/ a suitable cohort w/ enough cases to provide adequate statistical power
- Define criteria to identify outcome of interest
- Identify individuals in the cohort that developed the outcome of interest (called "cases")
- Select individuals that were also part of the cohort that haven't developed the outcome of interest ("controls")
- Retrieve samples or records from before outcome (potential predictor variables and compares levels between cases and controls)
35
When is a variable considered a confounder?
If it:
- Is independently associated w/ outcome
- Is associated w/ exposure
- Doesn't lie in the causal pathway
36
When does unmeasured confounding occur?
Occurs in a variable that wasn't measured during the course of the study
37
Can bias be completely eliminated?
No, only minimized
38
What is recall bias?
People remember things more readily when they have a negative outcome (especially when recalling exposures)
39
What is selection bias?
Selecting groups from different places
40
What is channeling bias?
Giving drug to sicker px
41
What is misclassification bias?
- Outcome/ exposure misclassified
| - Cases and controls were erroneously classified
