The marked item technique: a practical procedure for litter control.

Unobtrusively marked items of litter were placed among existing trash on the grounds of a federal youth correctional facility. Inmates voluntarily collected trash and deposited it at a central location, where they were given money or special privileges for each piece of marked litter found. A multiple-baseline design with litter counts in three areas revealed successive reductions of 55%, 88%, and 71% after 17, 22, and 36 days of baseline, respectively. A fourth area served as a baseline-only control, and revealed no systematic changes. Advantages of the procedure over previously devised techniques were discussed and applications in other areas of pollution control suggested.