Context : The study of the Milky Way relies on our ability to interpret the light from stars correctly .
With the advent of the astrometric ESA mission Gaia we will enter a new era where the study of the Milky Way can be undertaken on much larger scales than currently possible .
In particular we will be able to obtain full 3D space motions of red giant stars at large distances .
This calls for a reinvestigation of how reliably we can determine , e.g. , iron abundances in such stars and how well they reproduce those of dwarf stars .

Aims : Here we explore robust ways to determine the iron content of metal-rich giant stars .
We aim to understand what biases and shortcomings widely applied methods suffer from .

Methods : In this study we are mainly concerned with standard methods to analyse stellar spectra .
This includes the analysis of individual lines to determine stellar parameters , analysis of the broad wings of certain lines ( e.g. , H \alpha and calcium lines ) to determine effective temperature and surface gravity for the stars .

Results : For NGC 6528 we find that [ Fe/H ] = +0.04 dex with a scatter of \sigma = 0.07 dex , which gives an error in the derived mean abundance of 0.02 dex .

Conclusions : Our work has two important conclusions for analysis of metal-rich red giant branch stars .
1 ) For spectra with S/N below about 35 per reduced pixel [ Fe/H ] become too high , 2 ) Determination of T _ { eff } using the wings of the H \alpha line results in [ Fe/H ] values about 0.1 dex higher than if excitational equilibrium is used .
The last conclusion is perhaps not surprising as we expect NLTE effect to become more prominent in cooler stars and we can not use the the wings of the H \alpha line to determine T _ { eff } for the cool stars in our sample .
We therefore recommend that in studies of metal-rich red giant stars care needs to be taken to obtain sufficient calibration data in order to be able to also use the cooler stars .