56 



BREEDING CROP PLANTS 



four plots. Using this figure as a basis, a direct comparison 

 may be made between the average yield of any two strains in 

 terms of their probable errors. To carry our hypothetical 

 problem still further, suppose the average yield of four plots of 

 strain A was 20 bu. while that of strain B was 26 bu. As the 

 probable error of each average is 2 bu., and the probable error of 

 a difference is equal to the square root of the sum of the squares 

 of the probable errors of the two quantities, we would have 



26 2 



20 + 2 



6 + V^M or 6 2.8 

 The difference is only a little more than two times its probable 



TABLE X. PROBABILITY OF OCCURRENCE OF STATISTICAL DEVIATIONS OF 

 DIFFERENT MAGNITUDES RELATIVE TO THE PROBABLE ERROR 



