Perceptual and Motor Skills, 2009, 109, 3, 831-840. © Perceptual and Motor Skills 2009

DOI 10.2466/PMS.109.3.831-840 ISSN 0031-5125

1Address correspondence to Umberto Cesar Corrêa, Escola de Educação Física e Esporte, Universidade de São Paulo, Av. Prof. Mello Morais, 65, São Paulo SP, CEP 05508-030, Brazil or e-mail (umbertoc@usp.br).

RELATIVE FREQUENCY OF KNOWLEDGE OF RESULTS AND TASK COMPLEXITY IN THE MOTOR SKILL ACQUISITION1

DALTON LUSTOSA de OLIVEIRA

Universidade Presbiteriana MackenzieCentro Universitário Nove de Julho

UMBERTO CESAR CORRÊA

Universidade de São Paulo

ROBERTO GIMENEZ

Centro Universitário Nove de JulhoUniversidade Cidade de São Paulo

LUCIANO BASSO AND GO TANI

Universidade de São Paulo

Summary.—The aim of this study was to investigate the effects of knowledge of results (KR) frequency and task complexity on motor skill acquisition. The task consisted of throwing a bocha ball to place it as close as possible to the target ball. 120 students ages 11 to 13 years were assigned to one of eight experimental groups according to knowledge of results frequency (25, 50, 75, and 100%) and task com-plexity (simple and complex). Subjects performed 90 trials in the acquisition phase and 10 trials in the transfer test. The results showed that knowledge of results given at a frequency of 25% resulted in an inferior absolute error than 50% and inferior variable error than 50, 75, and 100% frequencies, but no effect of task complexity was found.

It is well known that practice is fundamental for the acquisition of motor skills (Corrêa & Tani, 2005). However, practice must be optimized if efficiency of the learning process is the goal. The present study inves-tigated one of the most important factors for the optimization of learn-ing: knowledge of results (Salmoni, Schmidt, & Walter, 1984; Swinnen, 1996). It is a form of feedback that informs the learner about the result of a movement. Information related to movement execution is usually avail-able to the learner through his own sensorial sources (intrinsic feedback). However, there are situations in which that information is absent or, when present, the learner may have difficulties in using it. In those conditions, an external source becomes necessary to provide the learner with that in-formation (extrinsic feedback or knowledge of results).

Knowledge of results can be provided in many ways by manipulating its frequency, precision, and temporal characteristics. Recently, frequency of knowledge of results (number of knowledge of results supplied in re-lation to the total number of trials) has been considered one of the most important variables that affect the acquisition of motor skills and, as such,

D. L. de OLIVEIRA, ET AL.832832

has received the attention of many researchers (Chiviacowsky, 2000; An-derson, Magill, & Sekiya, 2001; Bruechert, Lai, & Shea, 2003; Tani, Mei-ra Junior, & Gomes, 2005; Palhares, Lage, Vieira, Ugrinowitsch, & Benda, 2006; Laguna, 2008). For a long period of time, it was believed that fre-quent knowledge of results yielded better learning (Bilodeau & Bilodeau 1958; Bilodeau, Bilodeau, & Schumsky, 1959). However, this scenario changed drastically after the publication of a seminal paper in 1984, in which those early studies were criticized because they did not use a trans-fer or retention test to separate the transitory effects of performance from more permanent effects of learning (Salmoni, et al., 1984). In fact, many studies (Baird & Hughes, 1972; Castro, 1988; Wulf & Schmidt, 1989; Win-stein & Schmidt, 1990; Wulf, Lee, & Schmidt, 1994; Chiviacowsky & Tani, 1997; Wrisberg & Wulf, 1997; Lai & Shea, 1998; Bruechert, et al., 2003), us-ing the latter methodological approach, have found favorable results for low frequencies of knowledge of results, or at least have indicated that learning is not hindered by reduced frequencies.

These results have been interpreted differently by means of three hy-potheses: specificity, consistency, and guidance. The specificity hypothesis (Henry, 1968) refers to the similarity between the task practiced in the ac-quisition phase and in the retention test. In that sense, it questions the ex-perimental design in relation to the retention test (accomplished without knowledge of results), which could facilitate the task for the subjects who are already familiar with low knowledge of results frequencies (experi-enced during the acquisition phase). The consistency hypothesis (Win-stein & Schmidt, 1990) is based on the assumption that constant perfor-mance corrections induced by frequent knowledge of results could inhibit the acquisition of consistency in the execution of movement, which would make retention more difficult. Finally, according to the guidance hypoth-esis (Salmoni, et al., 1984), frequent knowledge of results could act as a guide for the learner toward the goal of the task during the acquisition tri-als. This orientation could generate a certain amount of dependency of the learner in relation to external information, inhibiting or interfering with other processing activities such as detection and correction of errors and elaboration of the motor plan (Schmidt, 1991).

Most of the studies on knowledge of results frequency were carried out using relatively simple tasks in a laboratory environment. However, Godinho and Mendes (1996) and Swinnen (1996) have pointed out the ne-cessity of more studies which focus on complex tasks and pay more atten-tion to ecological validity. The relationship between frequency of knowl-edge of results and complexity of the task was initially explored in studies that used summary knowledge of results. Schmidt, Young, Swinnen, and Shapiro (1989), using a simple task, found better results for large num-bers of trials summarized. On the other hand, Schmidt, Lange, and Young

KNOWLEDGE OF RESULTS AND TASK COMPLEXITY 833833

(1990), in a study involving a more complex task, found favorable results with small knowledge of results summaries.

Guadagnoli, Dornier, and Tandy (1996; Exp. 2) compared the size of the knowledge of results summary, the complexity of the task, and the learning phases. The results showed an interaction between the size of the summary and the complexity of the task only for the groups at initial phases of learning. In relation to the groups at advanced phases, the inter-action between summary of knowledge of results and complexity of the task was only partially confirmed.

Using a task that involved the control of several degrees of freedom, Wulf, Shea, and Matschiner (1998; Exp. 2) investigated the frequency of knowledge of results effects (control, 100, and 50%) in the learning of a sla-lom movement in a ski simulator. The results showed better learning for the 100% frequency group in relation to 50% and control groups. In anoth-er study with variations in task complexity and arrangements of knowl-edge of results controlled by the experimenter as well as self-controlled, Chiviacowsky (2000) found no interactions. However, as the amount of practice differed for the groups, the analysis of the true effects of the task complexity was difficult to measure.

In summary, the effect of relative frequency of knowledge of results considering task complexity on motor skill acquisition has not been thor-oughly studied yet. Therefore, the aim of the present study was to inves-tigate the hypothesis that for tasks with high complexity, high frequen-cies of knowledge of results will be necessary. Since task complexity is defined by the number of elements and their interactions, complex tasks would require higher motor control, and as a consequence the possibility of performance errors increases. This could imply the need for more cor-rections and, therefore, demand for more information about the results of the movement (higher frequencies of knowledge of results).

Method

ParticipantsOne hundred and twenty children who had no experience with the

experimental task, both boys (n = 60) and girls (n = 60), with ages between 11 and 13 years (M = 11.8, SD = 1.2), voluntarily participated in this experi-ment. The children were all students of a private school in São Paulo, Bra-zil. The participation required the written consent of those responsible for the children. Children in this age range were chosen as participants in-stead of adults in order to reduce the “experience effect,” but assuring that they were able to learn the experimental task.

Task and ApparatusThe tasks consisted of two bocha game throwing patterns. It consisted

D. L. de OLIVEIRA, ET AL.834

of throwing the ball (bocha) and placing it as close as possible to the tar-get (bolim).

The simple task consisted of throwing the ball with a backward-for-ward pendulous movement of the extended arm. The complex task was performed by the same pendulous movement followed by an overhead circular movement of the arm, thus involving the control of a greater num-ber of muscles, that is, degrees of freedom.

The dependent variable measure was the distance from the target in centimeters. The balls to be thrown were made of polyester (weight = 100 g; diameter = 5 cm). The target consisted of a small ball weighing 500 g and measuring 8 cm in diameter. Both balls were those used in the offi-cial Bocha game. During the trials, a mobile fence was used to block the ball trajectory from the students’ line of vision and prevent knowledge of results. The mobile fence was made of black fabric and metal connecting rods (1.70 m high, 1.50 m wide). Procedure and Design

The subjects were randomly assigned into eight groups (ns = 15) ac-cording to frequency of knowledge of results (25, 50, 75, and 100%) and complexity of the task (simple and complex). In the acquisition phase the subjects performed 90 trials. The transfer test was conducted 10 min. after the end of the acquisition phase and consisted of 10 trials without knowl-edge of results. The interval between the trials was approximately 30 sec.

The participants were requested to throw the ball, aiming to place it as close as possible to the target positioned at a distance of 12 m. After each throw, the experimenter positioned the fence in front of the subject in order to block the line of vision (see Fig. 1).

1 m 12 m

Mobile FencePlace of Throw Target

Fig. 1. Schematic illustration of the experimental environment

KNOWLEDGE OF RESULTS AND TASK COMPLEXITY 835

ResultsThe data were analyzed in blocks of 10 trials in terms of absolute

(Table 1) and variable (Table 2) errors. For the acquisition phase, analy-sis of variance (ANOVA) was carried out with repeated measures on the last factor, 2 (complexity) × 4 (frequency) × 9 (blocks), and for the transfer phase a factorial ANOVA, 2 (complexity) × 4 (frequency), was conducted.

TABLE 1Mean of Absolute Error (cm) in Blocks of 10 Trials, in the Acquisition (Blocks

1 to 9) and Transfer Phases (T), Independent of the Complexity of the Task

Group Acquisition Transfer

Task Freq. 1 2 3 4 5 6 7 8 9

Simple 100% 231 194 177 183 197 153 162 144 145 18175% 249 221 221 189 195 211 185 166 156 20650% 338 222 231 204 207 222 191 227 184 22825% 228 159 139 130 127 127 113 102 103 175

Complex 100% 266 243 240 265 257 227 213 217 193 22175% 303 183 144 155 135 148 98 108 92 15250% 332 203 244 204 203 177 191 184 183 22225% 250 152 157 150 133 131 131 125 114 168

Absolute ErrorFor the acquisition phase, the main effects for knowledge of results fre-

quency (F3,112 = 15.92, p < .01; η2 = 0.29), blocks (F8,896 = 41.30, p < .01; η2 = 0.26), interaction between complexity and frequency (F3,112 = 6.88, p < .01; η2 = 0.15), and interaction between frequency and blocks (F24,896 = 1.90, p < .01; η2 = 0.05).

In terms of knowledge of results frequency, the Tukey HSD test showed that 25% frequency was better than other frequencies (p < .05), and that 75% was better than 50% frequency. Regarding differences among blocks, the Tukey HSD test showed that the absolute error diminished in the ac-quisition phase (p < .05). In relation to the interaction between task com-plexity and knowledge of results frequency, the Tukey HSD test showed that 100% frequency in the complex task was superior to 100% frequency in the simple task (p < .05). It was also observed that in both tasks, 25% fre-quency had absolute error inferior to that in the blocks with 50% and 75% frequencies (p < .05). The Tukey HSD test also showed that in the complex task, 25% frequency had absolute error inferior to that of the blocks with 100% frequency. Finally, concerning interaction between knowledge of re-sults frequency and blocks, the Tukey HSD test showed that all the knowl-edge of results frequencies diminished the absolute error (p < .05).

In the transfer test, significant effects for knowledge of results fre�knowledge of results fre-fre-quency were found (F3,112  =  2.70, p < .05; η2  =  0.07). The Tukey HSD test showed that results were superior in the blocks with 25% knowledge of

D. L. de OLIVEIRA, ET AL.836

results frequency than in those with 50% knowledge of results frequency (p < .05). Variable Error

In the acquisition phase, there were significant main effects for kno�kno-wledge of results frequency (F3,112 = 39.91, p < .01; η2 = 0.51), blocks (F8,896 = 22.79, p < .01; η2 = 0.16), interaction between task complexity and know�), interaction between task complexity and know-know-ledge of results frequency (F3,112 = 5.85, p < .01; η2 = 0.14), and interaction between knowledge of results frequency and blocks (F24,896 = 1.56, p < .05; η2 = 0.04). The Tukey HSD test showed that performance in blocks with 25% knowledge of results frequency was better than that in blocks with other frequencies (p < .05), and that 75% knowledge of results frequency was better than 100% and 50% frequencies. In relation to blocks, the Tukey HSD test indicated that variable error in Block 1 was superior to that in the remaining blocks, and Blocks 2 and 3 had variable error superior to Blocks 7 and 9 (p < .05). There was a significant interaction between know�know-ledge of results frequency and task complexity. In the simple task, perfor-frequency and task complexity. In the simple task, perfor-mance was better under the 25% knowledge of results frequency than at other frequencies, and in the complex task, the 25% and 75% knowledge of results frequencies had variable error inferior to that at the 100% and 50% frequencies. With respect to the interaction between knowledge of results frequency and blocks, at all knowledge of results frequencies but 100%, variable error decreased over blocks (Table 2).

TABLE 2Mean of Variable Error (cm) in Blocks of 10 Trials, in the Acquisition (Blocks

1 to 9) and Transfer Phases (T), Independent of the Complexity of the Task

Group Acquisition Transfer

Task Freq. 1 2 3 4 5 6 7 8 9

Simple 100% 152 119 111 108 129 97 90 90 90 10175% 168 135 133 119 117 125 113 98 78 14550% 230 142 145 124 131 148 115 142 127 14925% 92 53 47 49 46 53 43 39 34 63

Complex 100% 180 146 140 172 162 133 126 135 114 13275% 207 106 73 82 76 77 40 49 43 8350% 243 160 207 120 125 112 121 129 109 14225% 126 63 52 58 51 52 49 48 42 65

In the transfer test, the main effect for knowledge of results frequency was significant (F3,112 = 7.98, p < .01; η2 = 0.17). The Tukey HSD test showed inferior variable error at 25% knowledge of results frequency than at oth-er frequencies.

Discussion The objective of this study was to investigate the effects of relative fre-

KNOWLEDGE OF RESULTS AND TASK COMPLEXITY 837

quency of knowledge of results and task complexity on motor skill acqui-sition. Overall, the results did not show an effect of task complexity. In the field of motor learning, task complexity has been traditionally classified in various ways; for example, perceptual complexity, decision complexity, and effector complexity (Billing, 1980). The concept of complexity is usu-ally defined as the number of parts or components of a task (Nussenzveig, 1999; Ward, 2002). In this study, throwing the ball with a pendulous move-ment followed by an overhead circular movement of the arm was consid-ered a more complex task than throwing the ball using only a pendulous movement of the arm, because it involved the control of a greater number of muscles. However, the results allow the speculation that the difference in complexity was not evident, that is, adding a circular movement actu-ally increased the effector complexity of the task but it did not result in a greater motor control demand because it was a well practiced movement.

In relation to knowledge of results frequency, the results showed that the subjects who practiced with 25% frequency obtained better perfor-mance in the transfer test. The subjects of this group had higher accuracy than those of the 50% knowledge of results frequency group and more consistency than those of the groups experiencing 50%, 75%, and 100% frequencies. A possible explanation is that when knowledge of results is not always available, the learner involves a more active process of search-ing for information to relate it to intrinsic feedback and construct a ref-erence for correction of the task. In other words, reduced knowledge of results frequency exerts a small guiding function, contributing to the en-hancement of error detection and correction. What possibly occurs is that after the execution of a movement, the learner involves himself in a cogni-tive process to relate the following information: action goal, initial condi-tions, response specifications, sensory consequences, and movement out-come (Schmidt, 1975). The participant retains in short-term memory the results of the established relationships between these information sources when knowledge of results is available and then develops further control in the trials without knowledge of results in the following way: informa-tion related to initial conditions and action goal does not change from tri-al to trial; the relation between specifications of the response and sensory consequences strengthens in the trials with knowledge of results, which contributes to the refinement of corrections, making learning more effec-tive. When there is no knowledge of results available, the learner tends to maintain the relation between response and consequences in the next trials, until receiving knowledge of results again when able to evaluate whether the relationship is correct or not (Tani, et al., 2005).

Another important aspect that should be considered in this study is the refutation of the orientation hypothesis (Salmoni, et al., 1984). The ori-

D. L. de OLIVEIRA, ET AL.838

entation hypothesis states that frequent knowledge of results could guide the performance in the direction of the goal during acquisition and tends to deteriorate during transfer. The present study did not confirm that hy-pothesis. This is evident when the performance of the groups given 100% and 25% knowledge of results frequency in the acquisition phase is ana- knowledge of results frequency in the acquisition phase is ana- in the acquisition phase is ana-lyzed. As previously described, the group receiving 100% knowledge of results had poor performance during acquisition and maintained that per-formance in the transfer test; the group receiving 25% knowledge of re-sults, on other hand, had high performance in both acquisition and trans-fer tests.

The results of the present study did not show a linear relationship for the utilized knowledge of results frequencies (25%, 50%, 75%, and 100%). Linearity probably does not occur because, besides the frequency, the un-certainty (amount of information) is another important element that can affect the use of knowledge of results. In agreement with Shannon and Weaver (1949), the more unlikely the occurrence of a sign in a message, the more informative a posteriori is its occurrence. In this sense, the group re-ceiving 25% knowledge of results frequency worked under a lot of uncer-knowledge of results frequency worked under a lot of uncer- worked under a lot of uncer-tainty (the learner received knowledge of results only once in every four trials); the group receiving 50% knowledge of results frequency had more information and so less uncertainty, and so forth. In other words, knowl-edge of results issued at different frequencies supplies different amounts of information.

Finally, the present results should be interpreted with some caution because a delayed transfer test was not used. Despite some evidence re-garding similar effects of knowledge of results frequency and other vari-ables related to extrinsic feedback on immediate and delayed transfer tests (Vander Linden, Cauraugh, & Greene, 1993; Weeks & Kordus, 1998; An-derson, et al., 2001; Badets & Blandin, 2004; Ishikura, 2005; Palhares, et al., 2006), the results are still controversial, suggesting that the time interval before retention tests is a crucial manipulation when studying permanent changes in behavior.

REFERENCES

Anderson, D. I., Magill, R. A., & Sekiya, H. (2001) Motor learning as a function of knowledge of results schedule and characteristics of task-intrinsic feedback. Jour-nal of Motor Behavior, 33, 59-66.

Badets, A., & Blandin, Y. (2004) The role of knowledge of results frequency in learn-ing through observation. Journal of Motor Behavior, 36, 62-70.

Baird, I. S., & Hughes, G. H. (1972) Effects of frequency and specificity of information feedback on acquisition and extinction of a positioning task. Perceptual and Motor Skills, 34, 567-572.

Billing, J. (1980) An overview of task complexity. Motor Skills: Theory Into Practice, 4, 18-23.

KNOWLEDGE OF RESULTS AND TASK COMPLEXITY 839

Bilodeau, E. A., & Bilodeau, I. M. (1958) Variable frequency of knowledge of results and the learning of a simple skill. Journal of Experimental Psychology, 55, 379-383.

Bilodeau, E. A., Bilodeau, I. M., & Schumsky, D. A. (1959) Some effects of introduc-ing and withdrawing knowledge of results early and late in practice. Journal of Experimental Psychology, 58, 142-144.

Bruechert, L. M., Lai, Q., & Shea, C. H. (2003) Reduced knowledge of results fre-quency enhances error detection. Research Quarterly for Exercise and Sport, 74, 467-472.

Castro, I. J. (1988) Efeitos da freqüência relativa do feedback extrínseco na apren-dizagem de uma habilidade motora discreta simples. Dissertação de Mestrado. Unpublished master thesis, University of São Paulo, Brazil.

Chiviacowsky, S. (2000) Efeitos da freqüência do conhecimento de resultados contro-lada pelo experimentador e auto-controlada pelos sujeitos na aprendizagem de tarefas motoras com diferentes complexidades. Tese de Doutorado. Unpublished doctoral dissertation, Faculdade de Motricidade Humana, Universidade Técnica de Lisboa, Portugal.

Chiviacowsky, S., & Tani, G. (1997) Efeitos da freqüência do conhecimento de resul-tados na aprendizagem de diferentes programas motores generalizados. Revista Paulista de Educação Física, 11, 15-26.

Corrêa, U.C., & Tani, G. (2005) Estrutura de prática e processo adaptativo em apre-ndizagem motora: por uma nova abordagem da prática. In G. Tani (Ed.), Compor-tamento motor: desenvolvimento e aprendizagem. Rio de Janeiro: Guanabara Koogan. Pp. 141-161.

Godinho, M., & Mendes, R. (1996) Aprendizagem motora: informação de retorno sobre o resultado. Lisbon, Portugal: Edições Faculdad de Motricidade Humana.

Guadagnoli, M. A., Dornier, L. A., & Tandy, R. D. (1996) Optimal length for sum�Optimal length for sum-mary knowledge of results: the influence of task�related experience and complex-ity. Research Quarterly for Exercise and Sport, 67, 239-248.

Henry, F. M. (1968) Specificity vs generality in learning motor skill. In R. C. Brown & G. S. Kenyon (Eds.), Classical studies on physical activity. Upper Saddle River, NJ: Prentice Hall. Pp. 331-340.

Ishikura, T. (2005) Average knowledge of results schedule in learning of timing: in-fluence of length for summary knowledge of results and task complexity. Percep-tual and Motor Skills, 101, 911-924.

Laguna, P. L. (2008) Task complexity and sources of task�related information during the observational learning process. Journal of Sports Sciences, 26, 1097-1113.

Lai, Q., & Shea, C. H. (1998) Generalized motor program (GMP) learning: effects of reduced frequency of knowledge of results and practice variability. Journal of Mo-tor Behavior, 30, 51-59.

Nussenzveig, H. M. (1999) Complexidade e caos. Rio de Janeiro, Brazil: Editora da Uni-versidade Federal do Rio de Janeiro.

Palhares, L. R., Lage, M., Vieira, M. M., Ugrinowitsch, H., & Benda, R. (2006) �nowl�Knowl-edge of results�delay interval effects in the acquisition of a serial skill of different compatibility levels. Journal of Human Movement Studies, 51, 47-61.

Salmoni, A. W., Schmidt, R. A., & Walter, C. B. (1984) �nowledge of results and mo-tor learning: a review and critical reappraisal. Psychological Bulletin, 95, 355-386.

D. L. de OLIVEIRA, ET AL.840

Schmidt, R. A. (1975) A schema theory of discrete motor skill learning. Psychological Review, 82, 225-260.

Schmidt, R. A. (1991) Frequent augmented feedback can degrade learning: evidence and interpretations. In J. Requin & G. E. Stelmach (Eds.), Tutorials in motor neuro-science. Dordrecht, The Netherlands: Kluwer Academic. Pp. 59-75.

Schmidt, R. A., Lange, C., & Young, D. E. (1990) Optimizing summary knowledge of results for skill learning. Human Movement Science, 9, 325-348.

Schmidt, R. A., Young, D. E., Swinnen, S., & Shapiro, D. C. (1989) Summary knowl-edge of results for acquisition: support for the guidance hypothesis. Journal of Ex-perimental Psychology: Learning, Memory, and Cognition, 15, 352-359.

Shannon, C. E., & Weaver, W. (1949) The mathematical theory of communication. Cham-paign, IL: Univer. of Illinois Press.

Swinnen, S. P. (1996) Information feedback for motor skill learning: a review. In H. N. Zelazninik (Ed.), Advances in motor learning and control. Champaign, IL: Human Kinetics. Pp. 37-66.

Tani, G., Meira Junior, C. M., & Gomes, F. R. F. (2005) Frequência, precisão e localiza-ção temporal de conhecimento de resultados e o processo adaptativo na aquisição de uma habilidade motora de controle de força manual. Revista Portuguesa de Ci-ências do Desporto, 5, 59-68.

Vander Linden, D. H., Cauraugh, J. H., & Greene, T. A. (1993) The effect of frequency of kinetic feedback on learning an isometric force production task in nondisabled subjects. Physical Therapy, 2, 79-87.

Ward, L. M. (2002) Dynamical cognitive science. Cambridge, MA: The MIT Press.Weeks, D. L., & �ordus, R. (1998) Relative frequency of knowledge of performance

and motor skill learning. Research Quarterly for Exercise and Sport, 69, 224-230.Winstein, C. J., & Schmidt, R. A. (1990) Reduced frequency of knowledge of results

enhances motor skill learning. Journal of Experimental Psychology: Learning, Memo-ry, and Cognition, 16, 677-691.

Wrisberg, C. A., & Wulf, G. (1997) Diminishing the effects of reduced frequency of knowledge of results on generalized motor program learning. Journal of Motor Behavior, 29, 17-26.

Wulf, G., Lee, T., & Schmidt, R. A. (1994) Reducing knowledge of results about rela-tive versus absolute timing: differential effects on learning. Journal of Motor Behav-ior, 26, 362-369.

Wulf, G., & Schmidt, R. A. (1989) The use of generalized motor programs: reducing the relative frequency of knowledge of results enhances memory. Journal of Experi-mental Psychology: Learning, Memory, and Cognition, 15, 748-757.

Wulf, G., Shea, C. H., & Matschiner, S. (1998) Frequent feedback enhances complex skill learning. Journal of Motor Behavior, 30, 180-192.

Accepted November 13, 2009.