A fixed-effects model indicated no evidence of association between COMT genotype and P300 amplitude (k=6, Z=0·14, p=0·887) and latency (Z=0·14, p=0·892), although the direction of effect was consistent with increased amplitude and latency in the Val/Met+Val/Val group compared to the Met/Met group (d=0·02 and d=0·05 respectively). There was evidence of significant between-sample heterogeneity in the case of amplitude [χ2(5)=11·99, p=0·035] but not latency [χ2(5)=7·22, p=0·204]. These data are presented graphically in Fig. 3.
