Bottom Up Testing is an approach to integrated testing where the lowest level components (modules, procedures, and functions) are tested first, then integrated and used to facilitate the testing of higher level components. After the integration testing of lower level integrated modules, the next level of modules will be formed and can be used for integration testing. The process is repeated until the components at the top of the hierarchy are tested. This approach is helpful only when all or most of the modules of the same development level are ready.[citation needed] This method also helps to determine the levels of software developed and makes it easier to report testing progress in the form of a percentage.[citation needed]

What three components make up Bottom Up Testing?