A sigmoid curve

Figure 1 shows a sigmoid curve

"Sigmoid" means

resembling the lower-case Greek letter sigma

or the Latin letter S

The sigmoid curve appears frequently in the natural world

and is also called an S-shaped curve

It can be seen

in ancient decorative art, Greek vases and the Yin and Yang symbol

The shape has fascinated artists, scholars and craftsmen

for thousands of years

One example of a sigmoid curve

is a dose-response curve

The dose-response curve shows the effectiveness of medication

If the dosage amount is very low

a drug's effect is limited

If the dosage amount exceeds a certain amount

its corresponding effect will appear

If the dosage amount is too much

the effect no longer increases