Epidemiology 2 Flashcards
Define
Observational Studies
- Uncontrolled studies/natural experiments
- Viewed as less definitive than clinical trials
- By the end of experiment, cohort might flake → your sample size will be lower
What can/can’t you control in observational studies?
- Only thing you control is finding the boundary in which you will observe
- Cannot control certain factors (or confounders)
List
Types of Observational Studies
3
- Cohort studies
- Case-control studies
- Cross-sectional studies
Define
Cohort Study
- Starts with exposed & non-exposed group, both w/o disease
- Follow groups to observe disease incidence or mortality rates
- How many people develop the disease based on who are & who are not exposed?
In Cohort Studies, how are disease rates in both groups compared?
rate ratio or rate difference(??) lol
List
Types of Cohort Study
2
- Retrospective (past to present)
- Prospective (present to future)
Define
Retrospective studies
- Faster & cheaper
- Getting people now, and asking their past lifestyle
- Info on exposure & confounders from historical information
Define
Issues w/ Retrospective Studies
- Bias, will they tell you accurately? Or too much/too little/incorrect info?
- Di ka sure if your medical records are accurate
Define
Prospective Studies
- Take longer, more expensive
- Measure exposure levels & confounding variables at baseline
- Good when biological samples are required
- Unreasonable to keep historical biological samples
- To study diseases difficult to ascertain in retrospect
Define
Issues w/ Prospective Studies
- Lifestyle can change throughout the study: Takes longer, can be multifactorial
- Inconsistent statistically
- Tip: get too much at first, para kung umonti u arent screwed
List
What can cohort studies consider?
2
Cumulative incidence or risk (disease events per person)
* This person had disease event, this person didn’t
Incidence rates or mortality
rates (disease events per person per time)
* Over a month, x people had this disease
Cohort studies are good for ____ exposures & ____ diseases
rare exposures & common diseases
Define
Case Control Studies
- Start: diseased & non-diseased groups
- Look backward in time
- More subject to bias than cohort
- Case: has disease
- Control: no disease
- Compares odds of exposure in each group
Define
Recall bias
- Cases recall past exposures more often than controls
- typical of retrospective exposure assessment
- some people may recall more, some less
- no such thing as perfect recall
- Results biased towards finding association
Case Control studies are good for ____ exposures & ____ diseases.
why?
common exposures & rare diseases
- Since ure starting with a bias looking into the rare disease, might as well look into those patients
- Cohorting a rare disease needs a huuuge cohort
- Unicorn disease: hard to catch
Proportion Exposed vs Odds of Exposure
Proportion exposed = a / (a+b)
Odds of exposure = a / b
a is no. of exposed
b is no. of non-exposed
What conclusion can we draw when comparing odds of exposure and control?
odds of exposure > control → exposure is associated with disease
Describe
Cross-sectional Studies (Prevalence Studies)
- Measures exposure & disease at the same time
- Done when outcome of interest is subclinical/asymptomatic disease
- Difficult to determine whether exposure DID precede disease
- looking at 1 point in time & getting samples (bio & envi) in that point in time
- Tend to be weaker in design (compared to cohort & case-control)
Define
subclinical disease
illness that is staying below the surface of clinical detection
- “under” the clinic, Clinic doesn’t see it bc you don’t go to the hospital bc it’s not that bad
- Ex. blood has the lead due to low exposure, but won’t show disease associated with high exposure (neurotoxicity)
Relationship bet. Cross sectional studies vs. cohort or case-control studies
Cross-sectional is Often only possible design
* Can be confirmed in cohort or case-control studies
* Often first step before cohort or case-control studies
Define
bias
Distortion of the true relationship bet. exposure & disease
List
Kinds of bias
3
Selection Bias
Confounding Bias
Information Bias
Define
Selection bias
- Selecting a study population not representative of general population
- Cannot be corrected during analysis
- Easy to do, but difficult to correct
- Once you selected your sample, kahit anong sample test won’t explain why you selected your samples