ED50
ED50 is the dose at which a coin-flip describes whether the drug works: give it to a large group and exactly half of them show the defined effect. It is the population's middle ground — the dose that the typical person needs to respond.
ED50 comes from a quantal dose–response curve, where the endpoint is all-or-none and the y-axis is the cumulative percentage of individuals responding. Because the curve is symmetric and steepest at its midpoint, the 50 percent point can be read off most reliably, which is why the median dose, not the mean, is the standard summary. A smaller ED50 means greater potency, since less drug is needed to reach the 50 percent mark.
Be careful not to confuse ED50 with EC50. ED50 is a dose given to a population and counted as a fraction responding (quantal); EC50 is a concentration acting on one system measured as a fraction of the maximum effect (graded). They answer related but distinct questions and should not be used interchangeably.
If a hypnotic puts half of test subjects to sleep at 5 mg, its ED50 for the sleep endpoint is 5 mg; a drug with an ED50 of 0.5 mg would be ten times more potent.
Lower ED50 = more potent.
The '50' is statistical convenience, not biology — the midpoint of a sigmoid curve is where small dose changes give the most stable read of the population's central tendency.