3.4 Chance

Finally, the simplest model of development is one in which every frog has the same chance of growing a fifth leg:

A flow chart, moving from left-to-right. At left is a cartoon tadpole. A pair of diverging arrows point up-and-right and down-and-right The up-and-right arrow leads to label reading Probability = x. An arrow points right to a cartoon frog with five legs, over a label reading 5 legs. The down-and-right arrow leads to a label reading Probability = 1 - x. An arrow points right to a cartoon frog with four legs, over a label reading 4 legs.
When the variation in leg number results from chance, every individual has probability x of growing five legs and probability 1 - x of growing four.

Any difference between ponds in the frequency of five legs is due to sampling error.

Keep the four alternatives in mind as you design studies that will let you discover the cause of variation in your frogs.