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tokens: the 615460 Å doublet and the set of stellar parameters derived according to Carretta et al. (2006) prescriptions with those derived by Carretta et al. (2006) . The right panel shows the good agreement between the spectrum-synthesis-based abundances and the (615460 Å doublet) EW-based values, based on our own set of stellar parameters. Here only LTE abundances are considered. Overall, we can then conclude that the different methods explored so far for the derivation of stellar parameters and/or abundances lead to small systematic differences (within the associated errors in our case), while the errors associated with the EWs/line-fitting measurements are mostly responsible for the dispersions around these values. To complete our diagnostic tests, we have also evaluated the effects of deriving the stellar parameters spectroscopically on our final Na. The results of this test are summarised in the Appendix. 4. Observed Na distribution along the RGB and AGB in NGC 2808 4.1. [Na/H] versus [Na/Fe] Fig. 7 Abundance distributions of our complete (AGB RGB) sample. Left : [Na/H]; middle : [Na/Fe i ]; right : [Na/Fe ii ]. Symbols are the same as in Fig. 2 . The horizontal dash-dotted lines mark the critical values distinguishing the 1P and 2P stars following Carretta et al. (2009b) criteria (see the text). The typical error bars are shown at the right-bottom corner of each panel (considering the similarity of the errors of the GIRAFFE and UVES samples and the clarity of the figure, we only show the error bars for the GIRAFFE sample which is the largest sample here). By comparing the errors in our derived metallicities () and the star-to-star dispersions (), we find the intrinsic spread in [Fe/H] of NGC 2808 to be within , when considering only the GIRAFFE sample (representing the large majority of our dataset). The stellar population in NGC 2808 can thus be considered homogeneous in its Fe content within a few hundredths of , as also pointed out by Carretta et al. (2006) . Figure 7 shows these effects on the observed abundance patterns, where we present our final Na abundance distributions for AGB and RGB stars in NGC 2808 as [Na/H], [Na/Fe i ], and [Na/Fe ii ] versus . Overall, the differences among the three panels are minimal. However, because of the extra uncertainties associated with Fe i (due to the NLTE corrections) and Fe ii (due to the paucity of lines), we chose to use the [Na/H] ratio to discuss our derived Na abundance distributions along the RGB and AGB. 4.2. [Na/H] distribution A quick inspection of Fig. 7 (left panel) shows that the AGB and RGB samples in NGC 2808 have similar Na abundance ranges. A two-sided Kolmogorov-Smirnov (K-S) test confirms this: the K-S statistic D (0.268) and the -value (0.137) derived for the [Na/H] distribution both indicate that there is only weak evidence to reject the null hypothesis, i.e. the two samples have the same distribution. Examining in more detail the dispersions () and the interquartile range (IQR)