Built-in functions : trend function

trend(<input>,<time>[,<initial>])

New in v6.6

The trend function calculates the trend in input, based upon the value of input, the first order exponential average of input, and the exponential averaging time averaging time. trend is expressed as the fractional change in input per unit time. If you do not specify initial, trend substitutes the value 0 for the initial value of the trend.

The trend function is equivalent to the structural diagram and equations shown in this figure:

Compartment   Average input : 

    Initial value = input-(averaging*initial) (real)
Flow   change in average : 
    change in average =         (input-Average_input)/averaging (1/day) 
 Variable   averaging : 
    averaging =         Variable parameter (day) 
Variable   initial : 
    initial =         Variable parameter (real) 
Variable   input : 
    input =         Variable parameter (real) 
Variable   trend : 
    trend =         (input-Average_input)/(Average_input*averaging) (1/day) 

Example:

Yearly_Change_in_GNP = TREND(GNP,1,.04)

This equation calculates the annual change in the input GNP. It starts with an initial value of .04 (4% per year).