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Three methods are commonly used to handle the unobserved energy levels in calculating the properties of plasmas. The contribution of the levels may be simply ignored, or the energies of the levels may be estimated by either the hydro genic or the Rydberg-Ritz approximation. In this paper we compare the predictions of plasma properties obtained with each of these methods for eu and Al plasmas for temperatures of 10 000-50 000 K and pressures from 0.1 to 600 atm. The validity of simplifying assumptions used to handle the unobserved levels has been documented, and regimes where each method is most appropriate have been identified.• No energy levels listed in tables; calculated with a curve fit of the lower stages of ionization. 1 i03 J.