 Note that this treatment of fizzling requires that we include variables for whether the rocks hit the bottle. If we try to model this case using just three variables, \(\BT\), \(\ST\), and \(\BS\), we will incorrectly judge that Billy’s throw is a cause of the bottle shattering. This raises the question of what is the “right” model to use, and whether we can know if we have included “enough” variables in our model. Fenton-Glynn (2017) includes some discussion of these tricky issues.