focus of this paper is the relative effectiveness of the yield calculation methods, and the dependencies on varied astrophysical and mission parameters.* Nonetheless, at some point one must decide upon a yield goal: the number of exoEarth candidates required to achieve a particular science goal. Here we present a quantitative approach to defining the yield goal most appropriate for missions that seek to find Earth-like planets.

The parameter $\eta_{\earth}$ is often casually referred to as the fraction of stars that have Earth-like planets in their habitable zones. Prior to planning a direct imaging mission, however, we will likely only measure the sizes of a small number of these planets. In truth, $\eta_{\earth}$ reflects the fraction of stars that harbor Earth-sized planets in their habitable zones, an exoEarth *candidate*.

Some unknown fraction of these Earth-sized planets will be barren rocks, some fraction will have liquid water on their surfaces ($\eta_{\rm H_2 O}$), and some fraction may have biomarkers consistent with the presence of life ($\eta_{\rm life}$). Thus, if a mission’s science goal is to find Earth-*like* planets by searching for signs of water and/or life, the completely unconstrained quantities $\eta_{\rm H_2 O}$ and $\eta_{\rm life}$ will determine the success of the 