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Questionnaires are often applied in sports psychology to measure a person's trait or state. However, the extent to which the questionnaire captures differences because of trait or state influences is often unclear. The latent state–trait (LST) theory offers the opportunity to separate both variance sources. This separation allows estimating specific reliability coefficients.The present paper gives a theoretical introduction to LST and its basic ideas. Using a real data set with N = 156 athletes we exemplify the steps necessary to derive the LST coefficients. All athletes filled out a comprehensive inventory assessing competitive anxiety on two occasions.The data are analysed with structural equation models based on LST principles.The results confirm the questionnaire's trait saturation.Finally, results are discussed in light of practical and theoretical implications.