1. Instituto Nacional de Estadística, 28050 Madrid, Spain
2. Department of Marketing, Faculty of Statistics, Universidad Complutense de Madrid, 28040 Madrid, Spain
3. Department of Information Technologies and Systems, Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain
4. Institute of Artificial Intelligence, Faculty of Computing, Engineering and Media, De Montfort University, Leicester LE1 9BH, UK