I8S2 RT^RAI, KCOXOMICS 



earnings of metayers in Central Italy are collected from various authorities 

 and set out in Table V. If the object were to show the average earnings per 

 man under the metayage system in different parts of Central Italy, the figures 

 would not be comparable for in working out the available labour in each 

 group and reducing to " men units ", the factors employed were not 

 constant. But if the object were merely to show the relationship between 

 the earnings and the general fertility of the holdings the figures might be 

 taken as comparable. The Table shows a very large range of variation in 

 the earnings of metayers, for the metayage contracts tend to remain the 

 same even in districts where yields differ markedly. 



The homogeneity of the data must therefore be subordinated to the ob- 

 ject in view. If, for instance, the yield of wheat per unit is being considered 

 in countries which difl'er in regard to climate or other physical conditions, 

 the data may be looked upon as homogeneous and therefore comparable 

 if it is a question of shewing that the phenomenon is a variable one. On 

 the other hand the data will not be homogeneous if it is a question of apprais- 

 ing the values of the different systems of cultivation, for the yield does not 

 only depend on the skill of the farmer but also on the general physical con- 

 ditions of the surroundings. 



Errors of judgment of the kind are frequently committed especially 

 in dealing with questions of deforestation. The returns under a permanent 

 s^^stem of forest management are compared whit those whichcould be obtained 

 by farm crops, no account being taken of the fact that the latter may only 

 be transitory^ and grown at the expense of the accvimulated capital in the 

 land. Other examples of the same kind are the comparison of the wealth 

 of natioiis or individuals under different conditions of locality or time or the 

 comparison of wages when the economic conditions are not the same. etc. 



Passing on to the methods of classification, grading and comparison 



employed in the interpretation of data and to Galton's index of correlation 



it is pointed out that the statistical study of agricultural problems will only 



reveal gross influences at work, for any one mass of facts whatever its nattire 



is connected with a number of others and expresses therefore the result of 



the action of an indefinite number of variable factors. The correlation 



was determined between the yields of lucerne and the rainfall (Table VI) 



from data collected by the Agricultural Institute at Perugia, According to 



the formiila (i). 



14893.60 



Coefficient of correlation, r = . = 0.40. 



yi3584-33 X 75240.40 



or in other words, the yields of lucerne are largely controlled by the spring 



(i) Coefficient of correlation, »' = ^, ,, 



where r and y are the mean deviations in each series when the deviations are static. In cases 

 where they are dynamic the above deviations are replaced by the difference between each effec- 

 tive term and the corresponding term of the normal curve which is plotted by a special process 

 of interpolation. 



