I have transformed my data to be expressed as cost per day. This was to eliminate any seasonality type affects month over month due to holidays, etc.
What I would like to do now is remove the trend from this data set (upward trend) to calculate a historical average daily cost. I'd prefer to stick with an easier to understand method and have been trying to us a 2x12 moving average to calculate trend.
Using either the additive or multiplicative method then suggests subtracting or dividing the moving average from my observation. This then leaves a small residual amount.
The problem is, I am trying to find a historical average without the trend included. Is there a better way to go about doing this?