Human exposure induced by wireless communication systems increasingly draws the public attention. Here, an indoordown-link scenario is concerned and the exposure level is statistically analyzed. The electromagnetic field (EMF)emitted by a WiFi box is measured and electromagnetic dosimetry features are evaluated from the whole-body specificabsorption rate as computed with a Finite-Difference Time-Domain (a.k.a. FDTD) code. Due to computational cost, astatistical analysis is performed based on a surrogate model, which is constructed by means of so-called sparse polynomialchaos expansion (PCE), where the inner cross validation (ICV) is used to select the optimal hyperparametersduring the model construction and assess the model performance. However, the ICV error is optimized and the modelassessment tends to be overly optimistic with small experimental configurations. The method of cross-model validation is usedand outer cross validation is carried out for the model assessment. The effects of the data preprocessing are investigatedas well. Based on the surrogate model, the global sensitivity of the exposure to input parameters is analyzed from Sobol’indices.