International yearbook of education, Vol. XXX, 1968. 

 - Geneva :  Paris,  IBE,  UNESCO,  1969. 
 - 566 p. 
 - IBE publication ;  322. .
<br/><br/><label>No. de Clasificación Universal Decimal: </label>OC 37 (100) =20 "1968" INN Vol. 30