INTRODUCTION

The shooting down over Iraq of two U.S. Blackhawk helicopters by friendly fire on April 14, 1994, illustrates the critical importance of aircraft recognition in many military operations and raises several human factors issues. One issue pertains to how aircraft recognition should be trained. A practical question one faces when developing any training regime - whether it is classroom instruction or computer-based instruction - is how to schedule trials. Does the manner in which trials are presented affect learning? The well-known spacing effect suggests that it does.

Studies using diverse tasks - such as verbal memory tasks (Glenberg, 1979; Jacoby, 1978), motor skills tasks (Lee & Magill, 1983; Shea & Zimny, 1983, 1988), and complex psychomotor tasks such as Space Fortress (Shebilske, Corrington, & Jordan, 1994) - have shown that spaced practice is better than massed practice. The present study addresses the question of generalizing the spacing effect to recognition of aircraft.

Previous attempts to apply the spacing effect to aircraft recognition have not produced consistent results. Jarrard and Wogalter (1992) showed that presentation schedule interacts with same and novel views of test stimuli. Those persons who are shown novel-view stimuli performed slightly better with spaced presentation, whereas those shown same-view (i.e., previously observed) stimuli performed slightly better with massed presentation. However, Jarrard and Wogalter (1994) failed to replicate their previous findings: There was no effect of spacing. The failure to replicate is difficult to explain. It leaves the practitioner with no clear answers about the effect of schedule on recognition, and it discourages application of the spacing effect in this domain. In fact, the spacing effect has not been widely applied to any real-life learning environment (Dempster, 1988).

One reason the spacing effect has not been widely applied is the absence of a clear theoretical understanding of the effect. Many questions remain unanswered. What are the boundary conditions of the effect (task type, individual differences, length of the interval)? What are the critical properties of the "space" (duration, consolidation of memory, opportunity for rehearsal, the cognitive processes involved)? These questions must be addressed before practitioners can intelligently apply the spacing effect, and they can be addressed only through theory-based research. Thus although the present research focuses primarily on the theoretical accounts of the spacing effect in the context of aircraft recognition, it has practical implications for training. These implications include generalizing the spacing effect to aircraft recognition, identifying boundary conditions of the effect, and examining gender differences. Moreover, the present study will have practical value to the extent that it moves toward a better theoretical understanding of the spacing effect.

THEORETICAL EXPLANATIONS OF SPACING

Several related but distinctly different phenomena have been loosely referred to as the spacing effect. These spacing phenomena include the distributed practice effect, the contextual interference effect (Battig, 1966, 1979), the spacing effect, and the lag effect (Underwood, Kapelak, & Malmi, 1976). The distributed practice effect is defined here as the tendency of practice trials to produce better performance during acquisition, retention, and transfer when trials are temporally separated than when they are massed. The contextual interference effect is defined as the tendency of intertrial interference to inhibit performance during acquisition but to enhance later retention and transfer. Compared with blocked practice, random practice is believed to produce greater intertrial interference. The spacing effect is defined as the tendency of spaced presentation of stimuli to produce greater retention than does massed presentation. …