  This empirical study proposes a novel methodology to measure users' perceived
trust in an Explainable Artificial Intelligence (XAI) model. To do so, users'
mental models are elicited using Fuzzy Cognitive Maps (FCMs). First, we exploit
an interpretable Machine Learning (ML) model to classify suspected COVID-19
patients into positive or negative cases. Then, Medical Experts' (MEs) conduct
a diagnostic decision-making task based on their knowledge and then prediction
and interpretations provided by the XAI model. In order to evaluate the impact
of interpretations on perceived trust, explanation satisfaction attributes are
rated by MEs through a survey. Then, they are considered as FCM's concepts to
determine their influences on each other and, ultimately, on the perceived
trust. Moreover, to consider MEs' mental subjectivity, fuzzy linguistic
variables are used to determine the strength of influences. After reaching the
steady state of FCMs, a quantified value is obtained to measure the perceived
trust of each ME. The results show that the quantified values can determine
whether MEs trust or distrust the XAI model. We analyze this behavior by
comparing the quantified values with MEs' performance in completing diagnostic
tasks.
