| note related to this contribution
| - Roberto E López-Martínez and Alma Rocha-Lackiz are in the Institute of Engineering, UNAM [Universidad Nacional Autonoma de México], Apartado Postal 70-472, 04510 México, DF; E-mail: lopmar@servidor.unam.mx; and lackiz@servidor.unam.mx. The authors wish to thank José Luis Fernàndez Zayas, Jane Russell and an anonymous referee for their valuable comments and suggestions for this paper. They are also indebted to the researchers and directors of UNAM who kindly participated in this survey.
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