68 



Afiricul/ural Research and Prochulivity 



iniroduciion of Po/ (regressions 2, 4), the marginal contribution 

 of the stock of knowledge becomes 



3p/ . , 



(4.12) 



The negative c value in wheat, as reported in table 4.1, regression 

 2, indicates diminishing returns to research in countries that ex- 

 ploited their potential yield increases prior to the period studied. 

 This is apparently not the case in maize. In this crop, past yield in- 

 creases are positively correlated with the marginal productivity of 



Table 4.3; Maize: Yield Regressions (Equation 4.13, 

 Logistic Borrowing Function 4.7) 



Notes: 

 No. of observations: 989 (49 countries, 21 years, 40 missing observations) 

 For definitions of variables see notes to table 4.2 

 Deflators: 



regressions 1, 2 regional deflator 



regression 3, K(t) area, B(t), p(t) regional 



regressions 4, 5 none. 

 Country dummies: included in all the regressions. 

 In parentheses: t values. 

 (X, /3 values were estimated in regressions 2 and 5. 



I 



