The study reported in this article investigated the appropriateness of Mathematical Knowledge in Teaching of three pre-service primary school teachers (PSTs), teaching an informal statistical inference (ISI) lesson to primary school students. Using an ISI framework and the Knowledge Quartet framework, the presence and appropriateness of the PSTs' teaching actions were coded and categorized. The results showed that PSTs were consciously engaged in making inferences based on sample data. The PSTs struggled to correctly interpret students' conceptual input and to explain ISI, in particular, how generalizing from a sample is possible. Teacher education should focus on how PSTs can foster students' understanding of the logic of drawing conclusions about a population based on a sample.