  Context. The use of ratios of small to large separations as a diagnostic of
stellar interiors. Aims. To demonstrate that model fitting by comparing
observed and model separation ratios at the same n values is in error, and to
present a correct procedure. Methods. Theoretical analysis using phase shifts
and numerical models. Results. We show that the separation ratios of stellar
models with the same interior structure, but different outer layers, are not
the same when compared at the same n values, but are the same when evaluated at
the same frequencies by interpolation. The separation ratios trace the phase
shift differences as a function of frequency not of n. We give examples from
model fitting where the ratios at the same n values agree within the error
estimates, but do not agree when evaluated at the same frequencies and the
models do not have the same interior structure. The correct procedure is to
compare observed ratios with those of models interpolated to the observed
frequencies.
