This is fairly old news for people following n-back research, and over-emphasizes one particular study: there have been other studies, even just looking at post-Redick studies. From http://www.gwern.net/DNB%20meta-analysis :
Thank you for the link to your meta-analysis. That's a lot more helpful than the limited literature review presented in the paper I linked.
After reading your analysis, I am confused about how you determined that "that there is a gain of small-to-medium effect size." It seems like once you account for the passive placebo effect you actually showed that there is a small-to-non-existent effect.
This article critically examines previous studies that showed a link between working memory training (specifically via n-back training) and fluid intelligence, finding that the results may not have been as positive as reported owing to a number of factors including the use of a no-contact rather than active control group, and difficulty selecting tests that isolate the impact of working memory on fluid intelligence. The authors also present findings from a new study that show no improvement in fluid intelligence from dual n-back training, visual search training (active placebo) and no training (no contact placebo).
PubMed
Journal Challenged