The error‐related negativity (ERN or Ne) is increasingly being investigated as a marker discriminating interindividual factors and moves toward a surrogate marker for disorders or interventions. Although reproducibility and validity of neuroscientific and psychological research has been criticized, clear data on how different quantification methods of the ERN and their relation to available trial numbers affect within‐ and across‐participant studies is sparse. Within a large sample of 863 healthy human participants, we demonstrate that, across participants, the number of errors correlates with the amplitude of the ERN independently of the number of errors included in ERN quantification per participant, constituting a possible confound when such variance is unaccounted for. Additionally, we find that ERN amplitudes reach high consistency within participants at lower trial numbers, yet when comparisons between groups of participants are desired, increasing error‐trial numbers lead to higher statistical power. We derive concrete suggestions for specific types of analyses, which may help researchers to more effectively design studies and analyze error‐related EEG data with the most appropriate measurement technique for the question at hand and trial number available.