374.4
Erdos , Renée , 
Teaching by correspindence : a study undertaken by Renée F. Erdos . London ) Longmans ( París ) Unesco ( c 1967 ) 
xvi , 218 p. ilus. , ilus. , mapas , facsims , 23 cm. ( Unesco source bopok , 3 ) 
" Bibliography " : p. 213 - 214 .