Extreme-Ultraviolet spectral observations of \ion[Fe xiii] lines, obtained during the 1989 and 1995 flights of the SERTS instrument, are compared with line emissivities derived from two different sets of atomic data and transition probabilities. One dataset is based upon calculations involving the Close-Coupling approximation, and the other is based upon the Distorted Wave approximation. Emission line ratios that are insensitive to density and temperature are used to assess both the quality of the transition rates and possible line blending. Density-sensitive line ratios are used to measure the electron density. The comparison between the observed and the theoretical line ratios yields a set of lines, free of blends, that is recommended for plasma diagnostic studies. Both theoretical datasets yield the same set of lines, which includes wavelengths of 200.02, 201.13, 202.04, 203.16, 203.82, 209.67, 209.92, 256.43, 312.17, 312.87, 320.80, 348.18, 359.64 and 413.00 Å. Electron densities derived from line ratios calculated with each theoretical dataset differ by a factor =~ 2, comparable to the scatter in density measurements from other ions formed at similar temperatures, as well as to the uncertainties on some of the individual density measurements. This precludes determining which of the two datasets is more accurate. Ambiguities in a few of the line intensity ratios suggest that new, more accurate calculations of transition rates are needed.