I have monthly sales data for past 3-years, so 36 data points. Let's just say that the mean value is 50 units per month, and the standard deviation is 8.5
Now, for the next year the management has set the monthly sales target to be 75 units. Can I just pick the standard deviation from the historical data and use it for next year projection without any modification?
Note that previously the mean value was 50 and stdev was 8.5, but now the mean value is forced to be 75. Can I still use 8.5 as the stdev?
The stdev is required to calculate the amount of safety stock we should have in our inventory planning.