The results of International Atomic Energy Agency (IAEA) inter-laboratory comparison program reported in the literature during 1992 and 2004 were analyzed to estimate analytical uncertainty in the geochemical analysis of geothermal waters. Statistical data treatments for the detection and elimination of outliers and extremums were performed using five methods: (i) data visualization (or raw data statistics); (ii) mean ( x ) and standard deviation (s) calculation after eliminating outliers with x ± 2s ; (iii) median and median absolute difference (MAD); (iv) Huber method; and (v) single-outlier type multiple test method involving Dixon, Grubbs, skewness and kurtosis tests at 99% confidence level. The results are reported in scientific notation with one significant digit of uncertainty.