Non-response bias has long been a concern for surveys, even more so over the past decades with the increasing decline of the response rates. A similar problem concerns the surveys based on non-representative samples, convenience and cost-effectiveness of which has increased with recent technological innovations that allow to collect large numbers of highly non-representative samples via online surveys. These two cases may be considered jointly since in both it must be assumed that the bias is the result of a self-selection process and, for both, quality indicators are needed to measure the impact of this process. In this study we analyze, in different scenarios, the combined use of two indicators that have been suggested in the non-response context, but which may work as well for convenience samples.