Certainly this is a new term for Project Managers.

You may be familiar with the concept of "baselining" in which a project's data is stored so that you can compare what actually happens (also called the "track"), during a project with what you thought would happen when you planned it.

You typically store a "baseline" so that you could later see how the time and monetary budget have shifted from the budgeted to the actual.

Once you have baseline data, one could write simple formulas such as the following...

1. To calculate simple error you could use the formula:

     Actual Value - Budgeted Value

2. To calculate percentage of error, you could use the formula:

     (Actual Value - Budgeted Value) / Budgeted Value * 100

Now, typically programs like Microsoft Project and WBS systems store a single baseline, but experience in managing real-world projects has shown that projects don't have one single lifecycle, their plan typically gets redesigned several times along the way, so if you honestly want to track a project and calculate how budgetted vs actual changes over time, then the only real way you can trace and see what's happening in a project is "multiple baselines".

To use the baseline snapshot feature, once you're confident your plan is "sound"...click the little camera iconand a spreadsheet will be created that stores all the values for that project. This spreadsheet is named "Snapshot1", "Snapshot2"...etc.. depending on how many times you click the camera.

In this way you can do useful things such as pressing it once per month to store the "monthly project state". One idea of how to use multiple snapshots would be to use the "Export All" feature it to export all Snapshots to Excel and then use Excel's analysis tools to graphs changes in budget.

Click the film strip icon to see Snapshot1, then use the tab strip on the bottom to select the particular snapshot you'd like to view.

Follow the procedure above to view the snapshot sheet you want, then click View->Delete Snapshot. A dialog will ask you if you really want to do this. Click YES and the sheet will be removed.

Simple example of using a Snapshot in a Spreadsheet Formula

Step 1 - Take a snapshot of your project.

Step 2 - Go to the Project tab and select every item in the Duration Column.

Step 3 - Go Sheet1 and PasteLink all the items into a row.

Step 4 - Now manually edit them (click on the red square!)  to show a simple error calculation. For example if one cell contains the formula =Project!B1, then edit the formula to show =Project!B1-Snapshot1!B1

Step 5  - For each cell, change the cell type to "number". Each cell should show 0.00 now.

Now...go to your Task view or Project spreadsheet and change the duration value you're comparing. Pop back to Sheet1 and you'll see the error difference between actual "tracked" value.

Using a similar technique you might instead show the percentage of error... or any other comparison operation you dream up as being of relevance to your project.