Ok I have a set of dates, that are predicted on an excel spread sheet, using the average of how often the item is sold over a set period (say 4years) it uses information such as when it was last sold (in the form of a date) and the average of how ofter it sold over that period. What I want is to make the predicted date for the next sale, allowing for variation in the previouse pridictions and acutal sale dates.
For Example. item 1 sells on average every 56 days. the next predicted sale date is 31/1/06. over the last 5 predictions the differance has been 5,3,10,6 & 21 days (9 day average) before or after the prerdicted date in each case. Do i need to explain it in more detail? if so contact me. Can it be done?