Evidence - Gate-keeping (Judicial and Adjudicator). R. v. Oppong
In R. v. Oppong (Ont CA, 2021) the Court of Appeal considered the 'gate-keeping' role of judges:
 The trial judge then assessed whether the probative value of the proposed evidence outweighed its prejudicial effect. He recognized that the “gatekeeper function does not involve bright lines” and that “it requires an analysis of costs and benefits and sometimes does not yield a straightforward “yes” or “no” answer” (citing Abbey, at para. 79 and R. v. Sheriffe, 2015 ONCA 880, 333 C.C.C. (3d) 330, at paras. 101-103, 108, leave to appeal refused,  S.C.C.A. No. 299). The trial judge reviewed a number of case authorities with respect to the assessment of the costs and benefits of evidence relating to gangs: Abbey; R. v. Riley,  O.J. No. 1374 (Ont. Sup. Ct.); R. v. Williams, 2013 ONSC 1076. He noted that the assessment of the probative value of expert evidence involves consideration of the significance of the particular issue, as well as the reliability of the evidence, including the expert’s methodology, expertise and objectivity, while the cost of the evidence addresses the usual risk of prejudice in expert evidence, as well as the risk of complexity and the abdication of the jury’s function to the expert. The trial judge observed that proving criminal organization offences would inevitably involve bad character evidence. As such, steps must be taken to limit the evidence to what is relevant and necessary, and to give mid-trial and final instructions to place the evidence in context and obviate prejudice.