A large scale nationwide radon survey was conducted in Greek dwellings between 1994 and 2000. Twelve hundred passive CR-39 detectors were distributed and collected along with 963 filled in questionnaires. These were rechecked during 2012-13 to evaluate factors that potentially affect indoor radon concentrations, namely the factors (i) area (environment), (ii) building level-floor, (iii) ground type, (iv) basement, (v) building type, (vi) construction year, (vii) building walls contact, (viii) wall materials and (ix) floor materials. The questionnaires were prepared by the research team according to international standards. One-way and multivariate statistical methods were applied for the analysis, in specific (I) Linear Regression Analysis, (II) One way or multiway ANOVA, (III) General MANOVA, (IV) Stepwise Regression Analysis and (V) Principal Components Analysis. The results revealed that approximately 0.1% of the dwellings exhibited outlier radon concentrations. Noteworthy statistical correlations were detected between the measured mean annual indoor radon concentration levels and the factors (ii) (building level-floor) and (viii) (wall materials). Weak evidence was provided for the corresponding correlation with the factors (v) (building type) and (vii) (building walls contact). Minor was the association with the factors (iv) (construction year) and (ix) (floor materials). Significant differences were detected in the results of the applied statistical methods.
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