Analysis of Covariance is a statistical test used to assess whether the means of more than two groups are equal after accounting for the effects of one or more other predictive factors.  ANCOVA accomplishes a similar test to ANOVA, while also accounting for and removing the effects of other factors such as gender, age or pretest score that may influence the outcome you want to know about.

Example: This example is an extension of the example for ANOVA.  If, in a program evaluation, you want to compare an outcome (e.g., posttest knowledge of ecology) for three groups (e.g., students who did not visit your center, students who visited for one day, and students who visited for one week) AND you want to account for several other potentially important variables (e.g., grade level and pretest knowledge of ecology), you can use ANCOVA.  Using this technique, you can test whether mean knowledge of ecology scores for the three groups are the same – while accounting for the influence of grade level and pretest knowledge of ecology score.