# Add Prior Year Comparison to Service Summary¶

When you include *Prior year comparisons* on Service summary reports, the report includes the percentage increase or decrease over the same period in the prior year. This topic explains how Volgistics calculates these percentages.

Service summary reports may contain service information on a monthly, quarterly, or annual basis. For each period they may also include *prior year comparisons* that show what the volunteer service was for the same period one year earlier, and how much it has changed in terms of percentage. Such comparisons help you understand changes in volunteer participation.

The following illustrations show how prior year comparisons appear in three different Service summary reports: one monthly, one quarterly, and one annual.

## Monthly¶

In this example the report includes volunteer hours for January 2000. With prior year comparisons selected, the report also includes a column for the same period in the previous year (January 1999) plus a "Change" column that shows the percentage increase or decrease between the two months. As you can see, the report shows a 97.4% increase in volunteer hours at the "Info Desk" from January 1999 to January 2000, and a 35.7 percent increase in volunteer hours served in "Information Services" for the same period. Service summary reports may include a single month, or a range of months. If you choose to include prior year comparisons, the comparisons appear for every month in the report.

## Quarterly¶

The report in this example is quarterly. It includes volunteer hours for the first quarter of 2000 (January 2000 through March 2000). With prior year comparisons selected, the report also includes a column for the same period in the previous year (January 1999 through March 1999), plus a "Change" column that shows the percentage increase or decrease between the two quarters. Service summary reports may include a single quarter, or a range of quarters. If you choose to include prior year comparisons, the comparisons appear for every quarter in the report.

## Annual¶

The annual report shown here includes volunteer hours for all of 2000. With prior year comparisons selected the report also includes a column for the same period last year (all of 1999), plus a "Change" column that shows the percentage increase or decrease between the two years. Service summary reports may include a single year, or a range of years. If you choose to include prior year comparisons, the comparisons appear for every year in the report.

## How the math works¶

While you never need to perform any math yourself when using this report, you might want to understand how these percentages are derived. Volgistics uses standard math for calculating the percentage change in the two values. It subtracts the prior period value from the current period value, divides the result by the prior period value, and then multiplies that result by 100.

Here's an example: Suppose your report has 1,000 hours of service in January 2003, and 1,100 hours in January 2004. To calculate the percentage increase, the report starts by subtracting the prior period value (January 2003) from the current period value (January 2004):

1,100 - 1,000 = 100

The result is 100, which means there were 100 more hours served in January 2004 than in January 2003. Next, the report divides the result (100) by the prior year value of 1,000:

100 ÷ 1000 = 0.1

Finally, the report multiplies the decimal result by 100 to get the percentage:

0.1 x 100 = 10%

Since this value represents an increase in service, the report displays it as a positive value, like this:

+ 10%

Volgistics uses the same math when there is a decrease in service. Here's another example: Suppose you track how many courses volunteers instruct. Your report shows 250 'courses instructed' in 2003, and 200 courses instructed in 2004. Since the second value, 200, is smaller than the prior year value of 250 there is a decrease in service. To calculate this percentage decrease, the report starts by subtracting the prior period value from the current period value:

200 - 250 = -50

The result is negative 50, which means there were 50 fewer courses instructed in 2004 than in 2003. Next the report divides negative 50 by the prior year value of 250:

-50/250 = -0.2

Finally, the report multiplies the decimal result by 100 to get the percentage:

-0.2 x 100 = -20%

Since this value represents a *decrease* in service the report displays it as a negative value, like this:

- 20%

If your report has a non-zero number for the current period, but a zero for the same period in the prior year, the report always shows a 100% increase.

If your report has a zero for the current period and a non-zero value for the same period in the prior year, the report always shows a 100% decrease.