Often, you may want to compare year over year values. How much has our customer base grown over last year? How did sales in each quarter compare to sales in the same quarter last year? These types of questions can be answered using Year over Year Growth.

Tableau exposes Year over Year Growth as one option in the Quick Table Calculations. Here, for example, is a view that demonstrates Sales by QUARTER, along with the percentage difference in sales for a quarter compared with the previous year:

The second Sum(Sales) field has had the Year over Year Growth Quick Table Calculation applied (and the mark type changed to bar). You'll notice the >4 nulls indicator in the lower right, alerting you to the fact that there are at least four null values (which makes sense as there is no 2015 with which to compare quarters in 2016). If you filtered out 2015, the nulls would appear in 2016 as Table Calculations can only operate on values present in the aggregated data in the cache. Any regular filters applied to the data are applied at the source and the excluded data never makes it to the cache.

As easy as it is to build a view like this example, take care, because Tableau assumes each year in the view has the same number of quarters. For example, if the data for Q1 in 2015 was not present or filtered out, then the resulting view would not necessarily represent what you want. Consider the following, for example:

The problem here is that Tableau is calculating the Quick Table Calculation using an addressing of Year and Quarter and an At the Level of value of Year of Order Date. This works assuming all quarters are present. However, here the first quarter in 2016 is matched with the first quarter present in 2015, which is really Q2. To solve this, you would need to edit the Table Calculation to only use Year for addressing. Quarter then becomes the partition and thus comparisons are done for the correct quarter.

So what if you don't want to show 2015? Filtering it will cause issues for 2016. We'll look at Late Filtering later in this section. Another potential way to remove 2015 is to right-click the 2015 header in the view and select Hide. Hide is a special command that simply keeps Tableau from rendering data, even when it is present in the cache. If you later decide you want to show 2015 after hiding it, you can use the menu for the YEAR(Order Date) field and select Show Hidden Data. Alternately, you can use the menu to select Analysis | Reveal Hidden Data.