| Abstract: | Human services interventions are most rigorously evaluated with true experimental designs in longitudinal experimental field trials (LEFTs). However, differential self-selection or attrition often pose a serious threat to the LEFTs internal validity. This threat can be largely overcome by describing all conditions in advance to prospective subjects and securing their agreement to participate in and complete whichever condition is selected at random by a Lottery. This solution, however, in turn then poses the external validity problem that the program's effects on those who would participate in a Lottery may well be different than its effects on those who would participate in any single condition. In the present paper, we describe a new design, termed the Combined Modified Design, which assesses and overcomes this problem. This new design, in which a modified version of the Randomized Invitation Design (in which only one condition, assigned at random, is described to a potential subject, but outcome measures are obtained on everyone) is combined with the Lottery LEFT, is illustrated with a hypothetical example. |
