Table 2.

Table to compute χ² test score.

	Positive class	Negative class	Total
Feature fi occurs	α	β	α+β=μ
Feature fi does not occur	λ	γ	λ+γ=τ − μ
Total	α+β=ω	β+γ=τ − ω	τ

The sum of instances comprising feature f_i is denoted by μ, the sum of instances without feature f_i is denoted by τ − μ, the sum of instances that are positive is expressed as ω, and the sum of instances that are negative are represented by τ − ω. Let the observed values be α, β, λ, and γ with the expected values E_α, E_β, E_λ, and E_γ. Based on the hypothesis that the two events are independent, the expected value can be evaluated as follows: