1. Institute of Neurology, School of Medicine, University of Belgrade, Dr Subotića 6, Belgrade 11 000, Yugoslavia
2. Institute of Epidemiology, School of Medicine, University of Belgrade, Višegradska 26a, Belgrade 11 000, Yugoslavia
3. Institute of Biostatistics, Public Health and Research in Medicine, School of Medicine, University of Belgrade, Dr Subotića 15, Belgrade 11 000, Yugoslavia