PERIODICITY IN HUMAN BEINGS AND MICE 



807 



Important features of the periodogram are: (1) its maximum oc- 

 curs for tlie trial period which corresponds to the period of our 

 function studied, and (2) the value of its maximum is the amplitude 

 C of our function. For an arbitrary function, the periodogram may 

 exhibit any shape whatsoever. But if the periodogram has a well- 

 defined maximum point, the abscissa of this point may be taken as 

 the estimate of the period and the ordinate as the estimate of the 

 amplitude (Koehler et al., 1956). 



In dealing with such estimates, we must realize, of course, that they 

 represent descriptive rather than analytical statistics. The adequacy of 

 the description (i.e., the precision and sampling variability of the esti- 

 mates T and C) depends heavily upon the precision and fundamental 

 variability of the original measurements. The usefulness of periodo- 

 grams in exploring questions concerning both the reality of a perio- 

 dicity and the sampling behavior of the estimates of its period and 

 amplitude thus also depends upon the precision and basic variability 

 of the original measurements. Moreover, as has been emphasized 

 earlier, the uncertainties of repeated physiologic measurements also 

 must be considered in this connection. 



Let us now illustrate the application of the periodogram technique 

 to the study of the rhythm in body temperature of intact mice. Rectal 



o 



o0.8 



a. 

 6 



^0.6 



"5. 



I 04 



c 

 o 



eO.2 



o. 

 o. 



< 







22 



23 24 25 



Estimote of Period (hours) -f 



26 



Fig. 2. Periodogram computed from the mean rectal temperature data 

 of a group of male CBC mice (A/ = 4 hr, 7" = 412 hr). 



