Unfortunately, even if we only condition on pretreatment co- variates, we can still induce collider bias. Consider what would happen if we condition on the collider 𝑍 2 in Figure 4.16. Doing this opens up a backdoor path, along which non-causal association can flow. This is known as M-bias due to the M shape that this non-causal association flows along when the graph is drawn with children below their parents. For many examples of collider bias, see Elwert and Winship [19]