Types of Descriptive Studies
- Case reports/series
- Correlational (Ecologic)
Types of Analytical Studies
- Prospective Cohort
- Retrospective Cohort
- Randomized trials
Characteristics of Descriptive Studies
- Relatively inexpensive
- used when little is known about dz.
- less time-consuming
- easy to collect information
- info already exists
- info already exists
Pro's of Case Reports (Series)
- may be 1st indication of an epidemic or newly recognized dz.
Con's of Case Reports (Series)
- No control groups
- Can't generalize finidings
Characteristics of Cross-sectional Studies
(AKA Prevalence studies)
- snapshot of health @ a defined point in time
- Exposure & dz are assessed @ the same time
- Used to quantify magnitude of a problem
- Uses surveys
- Incidence rate cannot be measured
Pro's of Cross-sectional Studies
Cons of Cross-sectional Studies
Can't assess cause & effect
Which study types looks at Herd Immunity?
How is an ECOLOGICAL FALLACY committed in Ecologic Studies?
by assuming that the association found @ herd level is also true on the individual level
Pros of an Ecologic Study
- Observes patterns --> form hypotheses
- Quick & cheap --> uses available info
Cons of Ecologic Studies
- Can't link exposure to dz.
- Can't control confouding factors
Goal of descriptive studies
to describe the occurrence of disease in populations & generate hypotheses
Goal of Analytical Studies
- ID & explain the causes of disease
- Quantify the effect of a potential risk (exposure) factor
Characteristics of Case Reports
- Describes a single Case
- May lead to the formulation of a new hypothesis
- Often 1st report of a new disease
- reported as a clinical narrative
What is the minimum number of cases needed to be a "case series"
Why are Cross-Sectional Studies often performed?
to quantify the magnitude of the problem (dz)
Use of a 2x2 Table
- Most common way to measure the association bewteen an exposure factor (+ or -)
- Occurrence or absence of dz.
What is a cohort?
a group of animals who share a common experience with a defined time
(ex. birth cohort, year entered vet school)
What is the best method in comparing disease INCIDENCE between 2 cohorts?
Characteristics of Prospective Cohort Studies
- Select exposed & non-exposed groups
- Follow them forward & measure the amount of disease that occurs in each
- Follow-up is from present to future
Characteristics of Retrospective (HX) Cohort Studies
- Uses past reliable medical records
- selects groups according to presence or absence of exposure
- Traces groups to present to determine dz. or outcome status
- follow-up is from past to present
Relative Risk Ratio (RR)?
the ratio of the probability of an event occurring (for example, developing a disease, being injured) in an exposed group to the probability of the event occurring in a comparison, non-exposed group.
2 important features of Relative Risk Ratio
- Comparision of risk between two "exposures" puts risks in context,
- "exposure" is ensured by having proper denominators for each group representing the exposure
If RR is close to 1 what does that mean?
The exposure is probably not associated with the risk of the dz
If RR is greater or smaller than 1?
the exposure is likey to be associated with the risk of dz
The greater the departure from 1 of the RR?
The stronger the association between the risk & the dz.
How do you calculate RR?
Relative Risk = Incidence in the exposed
Incidence in the unexposed
RR= 1, NO ASSOCIATION
RR > 1, (+) association, possibly causal
RR < 1, (-) association, possibly protective
What do we use to determine the statistical significance of RR value?
What is the confidence interval a measure of?
a measure of the reliability of an estimate.