In addition to reporting the actual mean values obtained at the 

 different time intervals sampled, the technique of the moving average 

 (Longley-Cook 1970) was utilized. This is one method to determine the 

 trend in a time series; it tends to smooth out short-term variations 

 in the data giving it a smoother curve. The advantage of using this 

 technique instead of fitting a curve freehand is that it is free from 

 operator bias since a formula is used. In a series of data Xj, xj, 



Y -4- Y +Y Y +Y +Y 



X3...X , the moving average is calculated by 1 2 5 , 2 5 4 , etc. 



" 3 3 



The average value over a number of recordings is then substituted for 



each individual value. 



47 



