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Cohort (Follow-Up) Studies consists of what two types of studies
Prospective and Retrospective (Historical) Studies
Cohort Studies
Start With Exposed Group followed over a long period of time to determine disease status
Calculating the incidents rate
Prospective Cohort Studies
Starts with present and collects into the future
Follow-up over long periods of time to see how disease has developed
Retrospective Cohort Studies
Data has already been collected in the past
Used to study Occupational or Environmental exposures
Look at current health records to see who developed the disease
Advantages of Prospective Cohort Studies
1. Can determine incidence
2. Good for Rare Exposures
3. Can study multiple Outcomes
Disadvantages of Prospective Cohort Studies
1. Long period of Follow-up
2. Time Consuming
3. Expensive
Advantages of Retrospective Cohort Study
1. No long follow up period (disease already occurred)
2. Less expensive
3. No problems with non-participation
Disadvantages of Retrospective Cohort Study
1. Large number of subjects
2. Expensive
3. Tracking can be Time Consuming
Case-control
1. Has cases and controls
2. Identifies potential exposures back in time
3. Measures odds ratio
Case Control Advantages
1.Short/No period of Follow-up
2. Smaller number of subjects
3. Good for studying rare Diseases or those of long latency
4. Less time consuming, Less Expensive
Case control Disadvantages
1. Selection bias
2. Interviewer Bias
3. Less certain of exposure status
Clinical Trials
The investigators influence the exposure of the study subjects
Randomization and Blinding occurs
Field Trials
Community Trials
Advantages of clinical trials
1. Fewer Biases
2. Gold Standard
3. Control for unknown factors
Disadvantages of Clinical Trials
1. Complex Design
2. Expensive
3. Time Consuming
Cross-Sectional Studies
Designed to determine what is happening right now
Disease and exposure status are determined at the same time
Usually uses surveys and polls
Advantages of Cross-Sectional Studies
1. Inexpensive
2. No loss to follow-up
3. Used for public health planning
Disadvantages of Cross-Sectional
1. Only a snapshot
2. Can only determine prevalence
3. No information on cause-effect relationship
Prevalence Formula
PE=A/A+B PnE=C/C+D (Cross-Sectional)
Incidents Rate Formula
IE= A/A+B InE=C/C+D (Cohort Study, Clinical Trials)
Odds Ratio
Odds of Exposure D= A/C
Odds of Exposure ND=B/D
OR=AD/BC (Case Control)