## Experimental & Observational, (Analytical, Descriptive)Study Designs

**COHORT STUDY**- compare the
**exposure**(exposed vs unexposed) - to find
**probability of getting disease**(**or****disease incidence).** **Risk Ratio of ‘disease’ incidence**.

- compare the
**CASE-CONTROL STUDY**- compare the
**disease**(diseased vs healthy) - to find
**risk factor frequency**. **Odd Ratio of ‘exposure/risk factor**‘.

- compare the
**NESTED CASE CONTROL STUDY (NCC)**- A
**case-control study nested within a cohort**study. **exposure**of interest is**only measured among the***cases*and the selected*controls*.

- A
- Because the number of cases is probably fairly small, can match
**multiple controls to a given case to increase the power.** - NCC design
**is**often used when the**exposure of interest is difficult or expensive**to obtain and when the**outcome is rare.**- NCC is more efficient than the full cohort design.
**Cases of a disease**that occur**in a defined cohort are identified**and, for each, a specified number of**matched controls is**selected from among those in the cohort who have not developed the disease by the time of disease occurrence in the**case**.

- NCC is more efficient than the full cohort design.

##### Probability vs. Odds

**Probability**is essentially the same as**percentage**.- We tend to compare the number of occurrences of a certain
**outcome to the number of total events**measured. - Probability ranges
**between zero and one.** **Odds**is a ratio of the**likelihood of an event happening c**ompared to the likelihood of an event**not happening.**- Odds can be
**zero or any positive number**(not just values between 0 and 1).

###### How big should the RR or OR ?

- In
**RCT**we are satisfied with a**small increase (or decrease) in risk**. **Cohort study:****RR > 3**(for a minor adverse event)**Case-control**:**OR > 4**

## REFERENCES

- https://www.stomponstep1.com/odds-ratio-relative-risk-calculation-definition-probability-odds/
- https://www.slideshare.net/terryshaneyfelt7/what-does-an-odds-ratio-or-relative-risk-mean?from_action=save