Studying leadership is often interpreted as an intensely political activity, but it is also difficult because of the blending of leadership roles and responsibilities.

Or consider combinations of problems, such as these chemical infectors mixing with airborne pollens, and apparently pushing up incidences of asthma.

It is the combination of the last two factors an overabundance of banks with no sustainable way to turn a profit that constitutes the most serious and most difficult problem.

In the physical sciences, a partition coefficient (P) or distribution coefficient (D) is the ratio of concentrations of a compound in a mixture of two immiscible phases at equilibrium.