Particle clusters are meso-scale structures that develop during gas-solid fluidization. The solid holdup by the clusters is an important feature which greatly influences the momentum transfer between the gas and particulate phases. An existing cluster solid holdup, epsilon(sc), model was validated against solid volume fractions, epsilon(s), measured in two CFB test rigs by an optical fiber probe. Then, modeling of an identification factor, n, for cluster identification was studied by analyzing the features of the epsilon(s) time series data. The data showed that n correlates with two higher-order moments, the skewness and the kurtosis, which were then used to establish a model for n. The model shows that n is not a constant as commonly reported, but increases to a maximum and then decreases with increasing time-averaged local solid volume fraction. This cluster identification criterion based on the model for n verifies the accuracy of the existing epsilon(sc) model. (C) 2020 Elsevier B.V. All rights reserved.