I should probably also add the health warning that the delta-method approach to
ratios of coefficients is REALLY BAD unless the denominator is well away from
zero.
When the denominator is near zero nothing really works (Fieller's method is
valid but only because 'valid' means less than you might like for a confidence
interval method, and the bootstrap is useful but not necessarily valid)
-thomas
Thomas Lumley Assoc. Professor, Biostatistics
tlum...@u.washington.edu University of Washington, Seattle
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