Why is the scientific community calling out the FDA Chief and why is he apologizing?

The FDA( Food and Drug Administration) Chief ,Stephen Hahn said plasma gotten from people who recovered from the corona virus should be used in treatment and production of the vaccines.

This didn’t go well with other scientists and health workers as there was no sufficient data to ascertain the efficacy of the plasma of recovered patients and they rebuffed it which later made the FDA Chief to apologize.

the answer is true when observing or determining population size, a direct observation is use, in which each organism is counted in a population or in an area. one benefit of this measure is that you get the exact population count but it will take a long time to  determine  the  size  of a  population.