a-c. This example illustrates the effect of adding the covariate "previous injury" (Z
) to the statistical model used for the causal diagram in Figure 2a. Note that previous injury is associated with both warming up (through team motivation/aggression) and the outcome injury (through Contact Sport). After completing steps 1–4, one is left with figure 5b. Because previous injury (Z3) is included in the model, it has not been deleted from the causal diagram in Step 2, and one must join its ancestors (dotted line). Figure 5c represents the causal diagram after completing Steps 5–6. Because warm up is not dissociated from the outcome risk of injury in figure 5c, the statistical model that includes the covariates Z1, Z2, and Z3 will yield a biased estimate of warm up on the risk of injury.