SEMAINE D ETUDE SUR LE ROLE DE L ANALYSE ECONOMETRIQUE ETC. 
117. 
tion. This is true both of aggregate agricultural output and 
of the output of individual farm products. 
The factors that affect changes in output are often extre- 
mely complex and not too well understood. In fact, some 
are of a sort that are virtually unknowable. How do we know 
when a new variety of seed is to be available? How do we 
know how rapidly it will be adopted by farmers? How do 
we know how quickly it will be improved by more adequate 
adaptation to local conditions? In the United States we have 
learned a substantial amount about the answers to the last 
two questions from studies that have been made of the adoption 
of hybrid corn ('). While we knew that a new hybrid for 
another crop would probably be adopted much more quickly 
than the 13 years it took before corn hybrids changed from 
I to 809% of the seed used, we were not able to predict that 
the same change would occur in grain sorghums in about 3 
years. Nor did we predict that the yield advantage would be 
as great as it was. Our original expectation was that the yield 
differential between hybrid and ordinary grain sorghums 
would be about 25% ; the actual yield differential appears to 
exceed 50% and may be as much as 75%. 
The use of trends to project output appears to be of little 
value. Starting with the second decade of this century, the 
decade increases in U.S. farm output have been as follows 
(in percent): 
IQIO-IG 
IQIQ-2G 
1920-3 
G70 . 
(") Zvi GriLicues, Hybrid Corn: An Exploration in the Economics 
Technological Change, « Econometrica », 25 (a), October 1057 
7 
"161 Johnson - pag. 35