VI . C A L O R I M E T R I C M K A S U R E M E N T S 205 



is subtracted from the total variance. To make instrumental errors 

 negligible in this case they should not exceed 0.4 and 0.9%, respec- 

 tively. 



For human basal metabolism Berkson and Boothby (33) noted a 

 mean coefficient of variation of 4%. With the standardization pos- 

 sible in measurements with human beings, one is thus justified in do- 

 creasing the instrumental error of a respiration apparatus to 0.4%. 



If the result is the difference or sum of various figures, the contribu- 

 tion of the errors of the measurement to the error of the result is given 

 by the following formula: 



el = 26=^ (19) 



where c^ is the error of result attributable to summated error of 

 measurement, and e the error of each measurement. 



Even reasonably small relative errors may of course become very 

 serious when the result is a small difference of large items. If, for 

 example, the heat of combustion of food has an error of 2% and if the 

 gain in body substance amounts to only 10% of the energy in the 

 food, the error of the food analysis alone causes an error of 20% of 

 the result. 



In dealing with the accuracy of the result, one should keep in mind 

 the degree of accuracy w^arranted by the units in which the result 

 is expressed. If, for example, a 20% error is inherent in the definition 

 of an animal's surface area, not much is gained by improving the ac- 

 curacy of the metabolic rate 1%, and then expressing the result per 

 unit of the uncertain surface area. 



4. Errors in Interpretation 



In many cases, as in the one mentioned above, the errors of meas- 

 urement are far outweighed by mistakes of interpretation. Investiga- 

 tors in the field of animal bioenergetics rather frequently overlook 

 multiple correlation in their systems. One deduced, for example, from 

 his measurements, a relation between body size and metabolic rate of 

 rats, forgetting that the heavier animals were also the older ones and 

 that what he described as an effect of body size may have been an ef- 

 fect of age. 



5. Limitations 



Berthelot's hope of using heat of reaction as a direct index of chem- 

 ical affinity did not materialize. The heat of reaction does not permit 



