Context :

Aims : A spectroscopic analysis of stellar spectra can be carried out using different approaches ; different methods , line lists , atmospheric models , atomic parameters , solar abundances etc .
The resulting atmospheric parameters from these choices can vary beyond the quoted uncertainties in the literature .
Here we characterize these differences by systematically comparing some of the commonly adopted ingredients ; line lists , equivalent width measurements and atomic parameters .

Methods : High resolution and high signal-to-noise ( S/N ) spectroscopic data of one helium-core-burning red giant star in each of the three open clusters , NGC 6819 , M67 and NGC 188 have been obtained with the FIES spectrograph at the Nordic Optical Telescope .
The M67 target has been used to benchmark the analysis , as it is a well studied cluster with asteroseismic data from the K2 mission .
For the other two clusters we have obtained higher quality data than what had been analyzed before , which allows us establish their chemical composition more securely .
Using a line by line analysis , we tested several different combinations of line lists and programs to measure equivalent widths of stellar absorption lines to characterize systematic differences within the same spectroscopic method .

Results : The obtained parameters for the benchmark star in M67 vary up to \sim \SI 170K in effective temperature , \sim \SI 0.4dex in \log g and \sim \SI 0.25dex in [ Fe/H ] between the different tested setups .
Using the combination of equivalent width measurement program and line list that best reproduce the inferred surface gravity from asteroseismology , we determined the atmospheric parameters for the three stars and securely established the chemical composition of NGC 6819 to be close to solar , [ Fe/H ] =-0.02 \pm \SI 0.01dex .

Conclusions : We have highlighted the significantly different results obtained with different combinations of line lists , programs and atomic parameters .
The results emphasize the importance of benchmark stars studied with several different methods to anchor spectroscopic analyses .