Feb.4,i9i8 Variability of Yields of Fruit Trees and Field Trials 277 



arrangement. The mean of the "A" control plots is 1,367 pounds, that 

 of the "B " control plots is 1,389 pounds, yet, as shown in Table XIV, the 

 coefficient of variability is increased from 12.5^1.1 in "arrangement A" 

 to 18.2 ±1.6 in "arrangement B" for single plots, with similar increases 

 for the repeated plots. 



Table XIV. — Effect of two methods of arranging control plots 



The records of Stockberger's hop yields have been computed in the 

 same manner. The coefficient of variability of the plot yields in his 

 arrangement is 16.71 ±3.86. Moving the control plots down one — that 

 is, using plot A as c^ and so on — produces a coefficient of variability of 

 1 5.93 ±3.55. There is obviously no difference between these two values. 



The problem was also investigated by the use of the 8-tree plot lemon 

 records. In this case three possibilities were tried, with the first, second, 

 and third plots in turn as c^ and every subsequent third plot as a control 

 plot. The coefficients of variability for the different arrangements were 

 21. 7±2. 21, 22.5±2.29, and 23.8i2.82. In view of the probable errors 

 of the coefficients, there seems to be no real difference in the result of the 

 different arrangements in this grove. 



Since a decided difference was found in one case out of the three studied, 

 it would seem that there is a rather high probability that significant 

 differences may result from different arrangements of control plots. 



VARIABIIvlTY IN THIS YIELDS OF MORE THAN ONE YEAR 



It is often assumed that the mean vield of two or more years is less 

 variable than the yield of one year. 



We have had opportunity to study the variability in the yield of 60 

 navel-orange trees over a period of several years. The data were kindly 

 furnished us by Mr. A. D. Shamel, of the Bureau of Plant Industry. 

 The trees in question had been selected for their uniformly good produc- 

 tion and the individual yields recorded for six years. 



The figures presented in Table XV show that the variability of yields 

 fell off distinctly after one year, but the reduction was negligible after 

 the yields of two years were combined. It will be noted that the coeffi- 

 cient of variability of the single trees even for one year is notably low. 

 Considering the records of single trees, the average of six years' records is 

 not less variable than the average of two. 



