General format for a 2 X 2 table
| Sick | Well | Total | |
|---|---|---|---|
| Exposed | a | b | a+b = H1 |
| Unexposed | c | d | c+d = H0 |
| Total | a+c = V1 | b+d = V0 | T |
V1 + V0 = H1 + H0 = T
Internal validity: Internal validity examines whether the study design, conduct, and analysis answer the research questions without bias. Presence of bias will decrease the internal validity. External validity: It refers to the extent to which results from a study can be applied (generalized) to other situations, groups, or events. External validity will be low if the study group is not representative of the general population.
Reliability and validity are terms used in the context of laboratory and clinical studies and their findings.
Intention to treat analysis: It is a method for analyzing results in a prospective randomized study where all participants who are randomized are included in the statistical analysis and analyzed according to the group they were originally assigned, regardless of what treatment (if any) they received. It includes study participants who may have dropped off due to non-adherence to therapy.
| Disease | Present | Absent |
|---|---|---|
| Test + | a | b |
| Test - | c | d |
a = true positives
b = false positives
c = false negatives
d = true negatives
When two events, A and B, are mutually exclusive, the probability that A or B will occur is the sum of the probability of each event. When two events, A and B, are non-mutually exclusive, there is some overlap between these events.
Efficacy can be defined as the performance of an intervention under ideal and controlled circumstances, whereas effectiveness refers to its performance under ‘real-world’ conditions.