Olivia:
Efficacy and efficacious are very useful words. They are used to describe whether an action is "worthwhile." We have already established that it is feasible to go to the falls in one day. Now, I am just trying to ascertain whether it is worth it (given limited dollars and time). With both unlimited money and time, there is no need to be either efficient or efficacious.
edited to add:
I found this comparison of the two terms:
In general, efficacy is the ability to produce an effect, usually a specifically desired effect. For example, an efficacious vaccine has the ability to prevent or cure a specific illness. In medicine a distinction is often drawn between 'efficacy' and 'effectiveness'. Whereas efficacy may be shown in clinical trials, effectiveness is demonstrated in practice. The concept of 'self-efficacy' is an important one in the self-management of chronic diseases.
