|The experimenter's view of the trees.|
- Causal pathway redundancy: redundancy in causal pathways may mitigate the agent's effects on the downstream outcome of interest - blocking one intermediary fails because another pathway remains active
- Causal factor redundancy: the factor affected by the agent has both beneficial and untoward effects in different causal pathways - that is, the agent's toxic effects may outweigh/counteract its beneficial ones through different pathways
- Time dependency of the causal pathway: the agent interferes with a factor in the causal pathway that is time dependent and thus the timing of administration is crucial for expression of the agent's effects
- Multiplicity of agent effects: the agent has multiple effects on multiple pathways - e.g., HMG-CoA reductase inhibitors both lower LDL cholesterol and have anti-inflammatory effects. In this case, the agent may influence the outcome favorably, but it's a trick of nature - it's doing so via a different mechanism than the one you think it is.