Paired availability design is a design which can lessen selection bias in the situations where it is not possible to use random allocation of the subjects to treatments. The design comprises of three fundamental characteristics:

* The intervention is the availability of the treatment not its receipt;

* The population from which the subjects' occurs is well defined with little in- or out- migration;

* The study involves several pairs of control and experimental groups.

In the experimental groups, the new treatment is made available to all subjects though some might not receive it. In control groups, the experimental treatment is usually not available to subjects though some might receive it in special circumstances.