Based on observations obtained at the VLT Kueyen telescope and 3.6 m ESO telescope, Chile, the ESO programs Nos. 67.A-0280(A) and 72.D-0174A, respectively.

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Appendix A is only available in electronic form at http://www.edpsciences.org

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... $\Upsilon_{{\rm a},i}$

If we define $n_{{\rm a},i}$ as the total ion number density of element "a'' in the ith ionization stage ( $n_{\rm a} = \sum_i n_{{\rm a},i}$ ), then $\Upsilon_{{\rm a},i} = n_{{\rm a},i}/n_{\rm a}$ is the fractional ionization of ion $\{$ a, $i\}$ . For a given metallicity $Z_{\rm a}$ , $n_{{\rm a},i} = \Upsilon_{{\rm a},i} Z_{\rm a} n_{\rm H}$ .

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... treatments

The treatment is a particular combination of values of the factors involved in an experiment. The set of treatments is called the "experimental plan''. Experimental plans for any number of factors can be found in corresponding reference books (see, e.g., Johnson & Leone 1977).

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