Question about rate of change given a set of data

If you were given a set of data, such as population vs time, for example:

(years) 0, 10, 20, 30, 40, 50, 60

(population)5, 10, 20, 40, 80, 160, 320

Would you get the overall average rate of change by calculating each individual average rate of change and then averaging that? Or just by dividing the last population variable, 320, by the last time variable, 60?

(this was part of a functions chapter in my textbook)

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I think you would look at $$\frac{\Delta P}{\Delta t} = \frac{320-5}{60-0}$$

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This depends on the rate of change between which two times; Damien's answer generalizes. Given population value $P(t_i)$ and $P(t_j)$ , for times $t_i,t_j$ , i.e., P(0)=5, p(10)=10 , etc.(i.e., $t_i,t_j$ are values in {$0,10,20,30,40,50,60$}, i.e., $t_o$=0, $t_1=10$ , etc., the change between time $t_i$ and time $t_j$ (assume $t_i>t_j$ is:
$\frac {P(t_i)-P(t_j)}{t_i-t_j}$