How does ANCOVA differ from Blocking?

Both remove error from DV, but ANCOVA also controls for variation in IV associated with covariate.

As with blocking:


  1. The effects of the covariate are subtracted from the error term, making it smaller.
  2. The covariate is a more powerful way to do this if the control variable is continuous, but it's conceptually the same.

Unlike blocking:


  1. Treatment means are adjusted to account for differences on the covariate
  2. Random assignment to IV conditions normally prevent differences in covariate means (confounds should be designed out).
  3. But incase covariate does differ across groups, ANCOVA effectively partials out the effect of the covariate from the focal IV as well as the error term.

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