Feb.4.i9i8 Variability of Vields of Fruit Trees and Field Trials 279 



The variability of plots is based on hypothetical plots made up of one 

 tree from each of the treated and one from each of the untreated plots. 

 These hypothetical plots are therefore made up of an equal number of 

 trees having similar treatments. The variability of the 10 plots thus 

 obtained was computed for the single year 1910, and for the sum of two, 

 three, four, and seven years. Table XVII gives the coefficients of varia- 

 bility for single trees and for 5-tree plots. 



Table XVII. — Comparative variability of yields of Baldwin apple trees through a period 



of seven years « 



"Records taken from Hedrick (rpir, p. 172-174). 



It is interesting to note how slightly the variability of the yields is 

 decreased by combining two or more years. If regard is paid to the 

 probable errors, it can not be said that there is any real difference. In 

 other words, one year's records of the yields of these apple trees seem to 

 be as reliable for variation studies as those for several years. 



It seems, therefore, that the continuation through several seasons 

 may not so materially decrease the variability of tree 5aelds as one might 

 expect. This has a direct bearing on the reliability of the major portion 

 of the calculations of this paper which are based on the variability of the 

 yields of one season. It should be kept in mind, however, that these 

 studies do not concern the relative yield of one plot compared with another, 

 but rather deal with the total variation from the mean of the yields of 

 all the plots. As cited before from the work of several experimenters, 

 the relative productivity of a group of plots may not be fully determined 

 even after a period of years, whereas the tree yields from Hedrick, Shamel, 

 and the data of the writers indicate that a measure of the variability for 

 one year of a group of trees divided into plots, may be very representative 

 of the mean variability for several years. 



SUMMARY 



(i) The present paper is the result of a study of the nature and extent 

 of the casual variability of yields of fruit trees under field conditions and 

 its bearing on the reliability of plot trials. 



(2) Studies have been made upon the variability of the yields of orange, 

 lemon, apple, and walnut trees. The orchards studied were selected on 



27808°— 18 4 



