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The estimation of the excitation of voiced speech, or the glottal flow, is usually carried out using inverse filtering, where vocal tract resonances are cancelled from the speech signal. The parameters of inverse filtering are typically adjusted using subjective evaluations of estimated flow signals. Proposed is a method to assess objectively the quality of inverse filtered glottal flows. The method computes the group delay function over a single cycle of the estimated flow. Incorrect settings of the inverse result in the distortion of the group delay function. This distortion is easily detectable and can be used in a straightforward manner to quantify how accurately the zeros of the inverse filter match the poles of the vocal tract.