There are different ways to test for non-response bias. A common technique involves comparing the first and fourth quartiles of responses for differences in demographics and key constructs.[3] In e-mail surveys some values are already known from all potential participants (e.g. age, branch of the firm, ...) and can be compared to the values that prevail in the subgroup of those who answered. If there is no significant difference this is an indicator that there might be no non-response bias.