# Percentage Difference from Previous Value Analytic Function

This article describes how to use the Percent Difference from Previous Value analytic function in Arcadia Instant.

Create a basic line visual on the `World Life Expectancy` dataset, through an Arcadia data connection. See Basic Visual for Aggregates.

In our example, we compare the difference in population of Libya in the year 1965 and the previous year 1964.

1. Click the field on the measurement shelf, in this case, the `sum(population)` field on the Y Axis shelf.
2. In the Field Properties menu, expand Analytic Functions, and select % Difference from Previous.
3. In the Analytic Function: Percentage Difference from Previous Value modal window,

1. In Select aggregate for this measure, leave the default Sum. The other aggregation options are Count, Minimum, Maximum, and Average. In addition, some connection types also support Approximate Distinct Count and Exact Distinct Count.
2. Under Select entities over which to take % difference from previous value, select country. This is the grouping that applies when calculating the % difference from previous values.
3. Under Select the sort order, you can specify the order of sorting. This order specifies what the previous row is, and the row where the value is taken. If your visual already specifies sorting order, these values appear in the modal.

You can add additional sort parameters, by clicking the (add/plus) icon.

Here, edit the sort order to be: country ascending, followed by year ascending.

4. Under Offset, specify the number of previous rows over which the function executes. Analytic Function: Percentage Difference from Previous Value
4. Click Apply.
5. [Optional] If you examine the expression in the Enter/Edit Expression interface, it shows that the function has the following form:
```(sum([population]) - LAG(sum([population]), 1)
over (partition by [country] order by [country], [year])) / LAG(sum([population]), 1)
over (partition by [country] order by [country], [year])
as 'pct diff from prev(population)'```
6. Click Refresh Visual.

7. Notice the change in the appearance of the Y axis and the tooltip, from displaying the `sum(population)` in millions to `population` as a percentage difference from previous value.

Hover over any country to view the percentage difference of population from the previous year. In the following example, the tooltip shows an increase in the population of Libya from previous year 1964 to the current year 1965 by 3.92%. Percentage Difference from Previous Value Analytic Function Visual