Efficiency is doing things right; Effectiveness is doing the right things. - Peter Drucker
Efficiency takes what we know now and holds onto it. It's familiar and comfortable. By doing today, what looks very much like what you did yesterday; we keep ourselves safe and avoid failure.
Effectiveness is messy. It involves asking questions that people naturally shy away from. It's exploration of the unknown and therefore scary and risky. Failure is not only possible, it's common. From a short term view - it's unnecessary, unpleasant, and not worth the effort. From the long term view - It's absolutely vital, rewarding, and the only path to success.
Effectiveness lies on the other side of the chasm of complexity. But that's why it must come first, and must be revisited periodically. Otherwise you risk doing things really well, that don't matter at all. This is the worst kind of waste.
It's not enough to do your best. Find out the right thing to do, then do your best. - W. Edward Deming
Effectiveness, the hidden side of productivity that can no longer be neglected.