  We consider three recent large-scale calculations for the radiative and
electron-impact excitation data of N IV, carried out with different methods and
codes. The scattering calculations employed the relativistic Dirac $R$-matrix
(DARC) method, the intermediate coupling frame transformation (ICFT) $R$-matrix
method, and the B-spline $R$-matrix (BSR) method. These are all large-scale
scattering calculations with well-tested and sophisticated codes, which use the
same set of target states. One concern raised in previous literature is related
to the increasingly large discrepancies in the effective collision strengths
between the three sets of calculations for increasingly weak and/or high-lying
transitions. We have built three model ions and calculated the intensities of
all the main spectral lines in this ion. We have found that, despite such large
differences, excellent agreement (to within $\pm$~20\%) exists between all the
spectroscopically-relevant line intensities. This provides confidence in the
reliability of the calculations for plasma diagnostics. We have used the
differences in the radiative and excitation rates amongst the three sets of
calculations to obtain a measure of the uncertainty in each rate. Using a Monte
Carlo approach, we have shown how these uncertainties affect the main
theoretical ratios which are used to measure electron densities and
temperatures.
