cubic powers of $x$, $y$, and $\alpha$. We perform short runs of $10^4$ iterations for each set of parameter combinations. In particular, we evaluate the importance of each parameter in a simple iterative process: one parameter is added each time, and the one that yields the lower RMS is retained, if the RMS is lowered by at least 10 ppm. [(this arbitrary choice constitutes a limitation of the method)]{}. Examples of combinations are given in Table \[tab:parameter combinations\]. This empirical approach is useful to understand the NICMOS systematics. We find that $F_0$ is the most important parameter. The average flux varies by up to 0.6% between groups. These variations are also present in previous studies of NICMOS data [@Burke2010; @Gibson2011a], and their origin is not understood. Incidentally, $F_0$ also accounts for stellar variability between visits. The second most important parameter is the angle of the spectral trace, $\alpha$. In decreasing order of effect, we then find $x$ and $y$, then $w$ and $T$. Among higher order terms, only $\alpha^2$ yields a significant improvement. Including the HST orbital phase yields a negligible improvement and is not included. As seen in Figure \[fig:state parameters\], the HST orbital phase is in fact responsible 