Epidemiology Part 2 improved

What is a two-by-two table used for?

To organize exposure and disease data and compare risk between exposed and unexposed groups.

In a two-by-two table, what do rows and columns usually show?

Rows = exposure status; columns = disease status.

What does a represent?

Exposed and diseased.

What does b represent?

Exposed but not diseased.

What does c represent?

Unexposed but diseased.

What does d represent?

Unexposed and not diseased.

What are the row totals?

Exposed = a + b; unexposed = c + d.

What are the column totals?

Diseased = a + c; not diseased = b + d.

What is the total study population?

a + b + c + d.

What is risk?

The probability that people in a group develop the disease.

What does Re mean and how do you calculate it?

Risk in exposed group: Re = a / (a + b).

What does Ru mean and how do you calculate it?

Risk in unexposed group: Ru = c / (c + d).

What does P(D|E) mean?

Probability of disease given exposure

What does P(D|U) mean?

Probability of disease given no exposure.

In the example, what are Re and Ru?

Re = 200/1000 = 0.20 or 20%; Ru = 100/2000 = 0.05 or 5%

Absolute vs. relative measures?

Absolute = subtract; relative = divide.

What does an absolute measure show?

The public health impact of an exposure.

What does a relative measure show?

The strength of the relationship between exposure and disease

What is risk difference, RD?

RD = Re − Ru = [a/(a+b)] − [c/(c+d)]

In the example, what is RD and what does it mean?

RD = 0.20 − 0.05 = 0.15 or 15%; there is 15% excess risk associated with air pollution.

What do RD values mean?

RD = 0 no association; RD > 0 risk factor; RD < 0 protective factor

What is relative risk, RR?

RR = Re / Ru = [a/(a+b)] / [c/(c+d)]

What do RR values mean?

RR = 1 no association; RR > 1 risk factor; RR < 1 protective factor