You can compare your current Power Duration Curve data with historic training data. By default, your Power Duration Curve and Critical Power is based on the last 90 days of running data. More on this topic can be found here and here.
You might want to compare historic training data to current data to determine how you're performing relative to past training cycles or years, or to assess how accurate the data in your Power Duration Curve and Critical Power is for representing your current capabilities. The steps for comparing current data with past data can be found below:
1) Log into Powercenter on the web.
2) Navigate to the Profile section if not already visible.
3) Find the Power Duration Curve.
4) Select the "Compare to Date Range" option in the top of the Power Duration Curve section.
5) Select a date range of your choosing.
6) Scroll your mouse over the graph to compare your best effort power values of old and new for any duration.