Function to calculate %change
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This is the function used:
(it could be formatted as an equation like this also in the book, using $$ before and after)
Why the +1 at the end? Is it because the expected could be zero? The case when the expected approaches zero and the observed is >0 we will in any case see enourmous deviations in terms of very high or very low indicator values. For example, of the expected is zero and the observed is just 1 kg/km (i.e. not very much this either), the difference will be 100% and the indicator will show worst possible condition using the equatino above.
If there are a lot of areas with expected values equal to zero, then perhaps we can exclude these (set to NA) or somehow modify the equation, like using a conditional equation?