  We present an experimental and numerical study of missing-level statistics of
chaotic three-dimensional microwave cavities. The nearest-neighbor spacing
distribution, the spectral rigidity, and the power spectrum of level
fluctuations were investigated. We show that the theoretical approach to a
problem of incomplete spectra does not work well when the incompleteness of the
spectra is caused by unresolved resonances. In such a case the fraction of
missing levels can be evaluated by calculations based on random matrix theory.
