degree of freedom

The number of independent comparisons that can be made between the members of a sample of patients.

Note: This concept refers to the number of independent contributions to a sampling distribution (such as chi-square distribution). In a contingency table, it is the number of cells that can be filled freely, without changing the totals. In a table with i lines and j columns, ddl = (i – 1)*(j - 1) (for example, in a 2 x 2 table comparing two groups for a dichotomous outcome, such as death, the chi-square distribution associated with it has one degree of freedom).