Introduction:
It is quite clear that all managers and employees working in various organizations and companies are subject to performance appraisal. However, some of them probably have complaints that their appraisal was inaccurate and biased. In this regard, one of the possible cognitive biases is the halo effect error, in which a person's other characteristics and performances are judged based on one outstanding characteristic. Since one of the main performance appraisal methods in the workplace is assessment centers, this study aimed to compare the cognitive error of the halo effect among industrial and organizational psychology assessors and managers.

Method:
The research strategy was quantitative, and its method was experimental. 240 industrial and organizational psychologists’ assessors trained in assessment centers courses, and 145 managers’ assessors with at least five years of experience, participated in the research. The ROC curve and the area under the curve (AUC) were used to analyze the data. Also, the odds ratio in the correct classification of patient/non-patient and risk factor and non-risk factor indices, which were converted into a 2x2 psychologist/manager table, correct/incorrect classification, by an analogical transfer of the related characteristics, was used.

Result:
The results of the study showed that the assessments of industrial and organizational psychologists are more likely to fall within the correct classification of leadership competence compared to managers. Also, the odds ratio of correctly identifying leadership competence indicators in the industrial and organizational psychologists group is much closer to the odds ratio of the criterion assessors group. Moreover, the accuracy of industrial and organizational psychologists in the assessor role is close to that of the criterion assessors. Finally, industrial and organizational psychologists performed better in assessing leadership competency in assessment centers and had less halo effect cognitive error compared to managers.

Discussion:
The findings of this study can be considered and exploited within the framework of accreditation of assessment centers. It can help organizations design and implement the assessment centers process with greater confidence, maximize the validity of its results, and make more effective decisions with higher execution guarantees.