The next step within this analysis set involved an examination of differences among the groups in P300a and P300b amplitudes and latencies using a three factor MANCOVA. The analyses were not performed separately for each electrode site. Instead, a data reduction method was employed to reduce Type 1 error. A factor analysis was performed on P300 amplitude and latency across the sites to find topographic regions of homogeneity which could be reduced to a single score. A principal components analysis followed by varimax rotation yielded two factors (22) which explained most of the variance across the sites. The anterior sites exhibited greater loadings on the first factor. Sites around and posterior to the central sulcus exhibited greater loadings on the second factor. Highly similar factor structures were obtained for P300a and P300b. We therefore chose to calculate average scores representing P300 amplitudes and latencies within anterior (F3, F4, F7, F8, Fz, C3, C4, and Cz) and posterior (P3, P4, Pz, P7, P8, O1, and O2) regions. These regional values were incorporated into the MANCOVA as separate variates and individually tested
