Attrition, also known as dropout, occurs when participants fail to complete a post-test or leave a study after they have been assigned to an experimental group. It can lead to a biased estimate of the effect size because those that drop-out are likely to be different from those that stay in. For example, less motivated pupils or schools might be more likely to drop out of a treatment. A technique used to counter this potential for bias is “intention to treat” analysis, where even those that drop out of the treatment are included in the final analysis.