BACKGROUND This article explores the suitability of a proposed "Dual" model, in which both people and items are sources of measurement error, by assessing how the test scores are expected to behave in terms of marginal reliability and external validity when the model holds. METHOD Analytical derivations are produced for predicting: (a) the impact of person and item errors in the amount of marginal reliability and external validity, as well as the occurrence of "ceiling" effects; (b) the changes in test reliability across groups with different average amounts of person error, and (c) the phenomenon of differential predictability. Two empirical studies are also used both as an illustration and as a check of the predicted results. RESULTS Results show that the model-based predictions agree with existing evidence as well as with basic principles in classical test theory. However, the additional inclusion of individuals as a source of error leads to new explanations and predictions. CONCLUSIONS The proposal and results provide new sources of information in personality assessment as well as of evidence of model suitability. They also help to explain some disappointing recurrent results.

中文翻译：

---

人格测验分数的可靠性和外部有效性：人和物品错误的作用。

背景技术本文通过评估模型保持不变时的边际可靠性和外部有效性，评估了预期的成绩如何表现，从而探讨了拟议的“双重”模型的适用性，该模型中人和物品都是测量误差的来源。方法产生的分析推导用于预测：（a）人和物品错误对边际可靠性和外部有效性的影响，以及“天花板”效应的发生；（b）具有不同平均人为错误量的组间测试可靠性的变化，以及（c）差异可预测性的现象。两项经验研究也被用作说明和检查预测结果。结果结果表明，基于模型的预测与现有证据以及经典测试理论中的基本原理均相符。但是，将个人作为错误的根源又引起了新的解释和预测。结论该建议和结果为人格评估以及模型适用性提供了新的信息来源。它们还有助于解释一些令人失望的经常性结果。