"Collecting information about the performance of impairment testing proved extremely difficult because the field is so small. Only a handful of companies have ever marketed impairment testing systems and there is no list of their names. However, the Institute conducted an extensive networking program based on our contacts in the field that identified what we believe to be every company that has ever marketed impairment tests. There are only 10 such companies. Of these, only 6 manufactured systems for employers. Three of these 6 are now out of business. This means that there are only 3 companies currently in business that provide impairment testing systems for employers.
"By contacting these employers, we were able to identify 18 employers who had used impairment testing. Of these, 14 employers participated in our study. One employer that had used impairment testing is now out of business. The remaining 3 employers declined to participate."


National Workrights Institute, "Impairment Testing: Does It Work?" (Princeton, NJ: NWI, undated).