The present area of directly observable lava flows is estimated to be around 512,000 km2.

This is known as learning and is not directly observable.

For instance, he clarifies that performance does not have to be directly observable actions of an individual.

By focusing on directly observable behavior, teachers can avoid ambiguity.

Thus, it is important for the teacher to measure concrete, directly observable behaviors which can be counted and recorded.

Decision rules support decision-making, which produces a communication pattern that can be directly observable.

Many important changes, especially in the long run, would be neither directly observable nor easily measured.

Accumulation within just a few minutes at such patches is directly observable.

Again, there are reasons to believe that in this situation the risk free rate may not be directly observable.

The reference here is to the social form of the product, which is not directly observable.