This paper examines and builds on an earlier contribution to this journal focusing on private supplementary tutoring -- widely known as shadow education -- in Francophone West and Central Africa. Drawing on wider literature about research methods in this domain, it examines the basis for the numerical estimates presented in the original article and supplements those statistics with data from a subsequent survey. The paper stresses the significance of the topic, and highlights methodological lessons not only for these African settings but also more widely. In this way it contributes to further research agendas relating not only to private tutoring but also to methods in cross-national surveys of educational achievement.