PlanGuru's Trend based on historical method is great for situations where you want to forecast objectively based the trend of your historical data. When using this forecast method PlanGuru takes your historical data and finds a trend line by running a linear regression, and forecasts future amounts based on the trend.
There are three sub options under this method.
Based on the trend
This method forecasts each monthly amount based on the trend line constructed.
Evenly among interim periods
This method forecasts full year projected amounts based on the trend line, and allocates the trended full year amount evenly among each interim period.
Using seasonality factors designated during setup
This sub-method forecasts full year projected amounts based on the trend line, and allocates the trended full year amount to each interim period based in their seasonality percentages.
Requirements for using Trend Based on historical data method.
IF you're trying to use the trend based on historicals method, but don't have more than one year of historical data you will receive the following error message. In order for this Method to be accurate, PlanGuru needs a minimum number of data points. As a result you can only use this method of you have more than one year of historical data.
When using the Trend based on historicals method all of your historical periods should be filled in. When PlanGuru runs a linear regression it weights all periods equally, thus not filling in historical periods will distort your foretasted results. In the sample below I've left a bunch of periods open, and as a result my January forecast is way out of line with my December historical amount. This is because all of the 0 periods are factoring into the trend line calculation.