2 X 2 Table

Students who have been vaccinated with vaccine A were observed as having measles less frequently than those who are not vaccinated. If so, it might be inferred that vaccine A is effective against measles. If we observe that two variables are related in some way, we refer to them as being associated. In the vaccination example, one variable is the vaccination status and the other is the measles status. An individual can either have been vaccinated or not and subsequently either develop measles or not. It is common to present information about these variables in a table, called a contingency table.

The 2 x 2 table or contingency table is an important tool for an epidemiologist because it depicts the distribution of an exposure and an outcome in a population at the same time. A 2 x 2 or contingency table groups study subjects for many purposes. In many studies, the grouping is often based on disease and exposure status. Commonly we classify study participants as follows:

- A = number of people with both the disease and exposure
- B = number of people with the exposure but not the disease
- C = number of people with the disease but not the exposure
- D = number of people with neither the disease nor the exposure

2 X 2 Table Format
	Disease - Yes	Disease - No	Total
Exposure - Yes	A	B	A+B
Exposure - No	C	D	C+D
	A+C	B+D	A+B+C+D

Example

From a population of all teenagers at your school, a sample of 100 teenagers is selected. The hypothesis that a new supplement called CavAway prevents cavities is being tested. The hypothesis is tested by having these 100 teenagers take CavAway once a day for 6 months. After taking CavAway for 6 months, only 10 of the 100 teenagers developed cavities

- Can you describe how cavities were distributed among the 100 people who took CavAway?

Cavity Distribution
	Cavities	No Cavities	Total
CavAway	10	90	100

You may be wondering if the 90 students who did not develop cavities would have if they did not take the CavAway. Well, to further test our hypothesis, we would need to select a comparison group. The comparison group would also consist of 100 teenagers, but these students would not take the new supplement. Let's suppose we examined the comparison group at the end of the six month period also and found that 30 of the 100 teenagers developed cavities.

- Can you describe how cavities were distributed among the 100 people who did not take CavAway?

Cavity Distribution
	Cavities	No Cavities	Total
No CavAway	30	70	100

- A complete 2x2 table would look like this:

Cavity Distribution 2X2 Table
	Cavities	No Cavities	Total
CavAway	10	90	100
No CavAway	30	70	100

2 X 2 Table Activity

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