In order to anticipate problems, predict trends, and understand exceptions, a firm theoretical basis for the distance indicator is required. In the consideration of many distance indicators a theoretical understanding is all too often lacking. The ideal distance indicator should also be universally available. It should be found in spiral galaxies, ellipticals and irregulars, if clusters, groups and the field are to be equally sampled without bias. This universality constraint obviously weighs heavily against Population I distance indicators, because of the lack of any significant star formation in elliptical and S0 galaxies at the present time.
Singular events are to be avoided.
The hope is to find a distance indicator that has more than an occasional or unpredictably fleeting presence in a galaxy, but rather that it is abundant and always found whenever and wherever it is needed. Supernovae of all types suffer from this problem. Finally, the identification of the ideal distance indicator should be unambiguous. Two, three or more types and sub-types of the distance indicator, each having subtle differences in their properties inevitably require additional observations to resolve differences.