742 PONTIFICIAE ACADEMIAE SCIENTIARVM SCRIPTA VARIA - 28 
The first of these relations is usually used to calculate SR. 
In nominal value, the expression for national income is 
equal to 
° 
bh 
R 
at market prices, and to 
> y * 
at factor cost. But by (117-9) 
(117-11) 
4B  IBRj 
yY YR, 
whence it follows from (117-6) and (117-7) that 
(117-12) 
YbB = 1 SyY 
b 
If we consider here only the expression for national income 
at factor cost. we have 
(117-13) 
R 
= Yay 
[Ir] Allais - pag. 46