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.

Comparison between validity and reliability

Reliability or precision

Is it consistent?
It doesn’t matter if it is correct; if it consistently gives the same result, then it is reliable.

Validity or accuracy

Does it do what it is supposed to do?
How well does it do it?
Accuracy matters

Clinical versus statistical significance

Statistical significance is related to the probability of a study’s results being due to chance
Clinical significance refers to the magnitude and relevance of the actual treatment effect
With large sample sizes, even small treatment effects can appear statistically significant e.g. a decrease in blood pressure by 2 mm Hg may be statistically significant but clinically of no value

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.

How to increase the power of a study?

Increase the sample size
Increase the effect size*
Increase the alpha value
Decrease random error

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 and effectiveness

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.