Confounding

Confounding is an extreme case of a lurking variable that is perfectly correlated with an experimental factor. The effect of a confounded factor cannot be estimated so it is particularly important that confounding is avoided when data are collected.

Experiment with a new variety of wheat

The experiment was a waste of time and money — no conclusions can be reached about the new variety.

Trial of a new teaching method

The data do not allow you to reach any conclusions about whether the resource is effective.