Demet Yeşilbaş It is a common misconception in ERP research that the waveform peak is inherently meaningful. It isn't. There is nothing inherently meaningful about the size of the peak amplitude because what you're interested in is the underlying component, not the peak.

the peak amplitude ≠ the component

Additionally, the amplitude of a waveform will vary from trial to trial. It will also vary considerably from person to person and stimulus to stimulus. Lastly, it depends on which component one is examining. P300, N400, P600 components will tend to have larger amplitudes, whereas early components (P100, N100) will tend to be much smaller.

A smaller amplitude relative to other studies does not necessarily mean the difference is due to error. It could be the stimuli in your experiment do not elicit as large of electrophysiological response as those used in other studies. The absolute amplitude is generally unimportant. The relative average amplitude is the important feature of the data.

Dear Demet Yeşilbaş

It is all right. The ERP values are changeable due to the type and recurrent times of the stimuli. Different people show different ERP peaks as well.

Dear Demet Yeşilbaş,

You should take into account that to better visualise ERPs, it's a common procedure to perform a baseline correction (e.g., to substract pre-stimuli mean or to standarize post-stimuli period with pre-stimuli mean and standard deviation). This changes the units of the resulting signal.