Step 1: The two things every logic family fights over. Whenever engineers pick a logic family, they are always balancing two competing goals, how fast it switches, which is the propagation delay, and how much power it burns while doing so, the power dissipation. A family that is very fast usually pays for it with high power, and a very low power family usually pays for it with slower switching. Step 2: Why we need a single combined number. Comparing families using only speed or only power can be misleading, since one family might win on speed but lose badly on power. Engineers therefore combine both quantities into one figure of merit so different families can be ranked fairly on overall efficiency. Step 3: Writing the figure of merit. The accepted way to express this balance is as the ratio of propagation delay to power dissipation, $\text{FOM} = \dfrac{t_{pd}}{P_D}$. A smaller value generally indicates a better, more efficient family. \[ \boxed{\dfrac{\text{propagation delay time}}{\text{power dissipation}}} \]