Convenience sampling is one of the most commonly used sampling procedures in second language acquisition studies, but this non-random sampling procedure suffers from a lot of problems including the inability of controlling for initial differences between experimental and control groups. The present study tries to introduce conditions and criteria which enable researchers to account for these drawbacks and at the same time make validity claims. Individual scores and group statistics are compared with regard to a group of essential factors known to be important for the purpose of the study. The overall value calculated for essential factors is then used to make judgments about the groups' comparability. The contribution of this method to the current procedures of sampling arises from its factual accuracy which is supposed to enhance the validity of findings obtained from studies employing non-probability sampling procedures.