1. Introduction

2. VLBA observations
2.1. Sample selection
2.2. VLBA observation setup
2.3. VLBA data calibration and imaging

3. Model fitting and core shift measurements

4. Discussion
4.1. Typically measured values of the shift
4.2. Core position as a function of frequency
4.3. Can the same function describe shifts at all frequencies between 1.4 and 15.4 GHz?
4.4. Fitting results assuming opacity due to synchrotron self-absorption
4.5. Search for core shift variability

5. Summary