There are two methods of comparing measurements contained in a performance test run for the purposes of identifying performance trends: Compare to Baseline and Compare to Previous.

Comparison Method	Description
Compare to Baseline	You select one performance test run in the trend report and define it as the baseline. All measurements in the report are then compared to the measurements contained in the baseline.
Compare to Previous	All measurements in a performance test are compared to the measurements in the performance test run that immediately precedes it in the report.

It is important to understand the difference between the two comparison methods. The following example will illustrate how the same data can yield different results depending on the method you select.

As shown in the image below, the average Transaction Response Time measurement is being trended from four performance test runs: 3, 4, 5, and 6.

Performance test run (PT) 3 has been defined as the baseline (as indicated by the word Base in parentheses). The average transaction response times contained in the other performance test runs are compared to PT 3 only.

In PT 3, the average transaction response time for TRX_01 was 2.045. The average transaction response time for the same transaction in PT 5 was 2.035, which represents a slightly faster response time and therefore a slight improvement in the performance of this measurement. The percentage difference between the two figures is displayed in brackets, in this case -0.49%.

However, if the Compare to Previous comparison method was selected, then the average transaction response time in PT 5 would be compared not to PT 3, but rather to PT 4 (since 4 precedes it in the table). The value for PT 4 is 4.073 while for PT 5 it's 2.035, a percentage difference of -50.04%.

Using exactly the same data, the two comparison methods have yielded very different results. Only a slight improvement with the Compare to Baseline method (-0.49%), while a more significant improvement with the Compare to Previous method (-50.04%).

To identify significant improvements or regressions in performance, you can define unique thresholds to track differentials between measurements being compared. If a differential exceeds a defined threshold, that value appears in a predetermined color, identifying it as an improvement, minor regression, or major regression.

For example, if you define an improvement threshold for comparing transaction response times as 50%, then any transaction response time that is 50% lower than that of the baseline or previous run (depending on the comparison method) will appear in the color you defined for improvements.

In the example below, the following performance thresholds for the transaction response time (TRT) measurement have been defined:

• Improvement. At least 90% decrease

• Major Regression. At least 50% increase

These threshold definitions mean that any performance improvements or regressions which exceeds these percentages will be displayed in color, making them more identifiable.

In the following table, the Compare to Previous comparison method is used.

In the table above, we see that the value of the TRT measurement for the Action_Transaction in performance test run 4 is 46900% higher than in performance test run 3 - a performance regression which far exceeds the defined threshold for major regressions. Therefore, the value appears in red, the default color for major regressions.

The corresponding value for performance test run 5 represents a 100% improvement on performance test run 4. Since this percentage exceeds the defined threshold for improvements, the value appears in green, the default color for improvements.

The Custom Measurement Mapping feature allows you to reconcile inconsistent transaction or monitor names between performance test runs, thereby allowing you to properly trend these measurements.

The following are two examples of when you would use the Custom Measurement Mapping feature:

• Inconsistent transaction name

You run a performance test that contains the transaction BuyBook. A while later you run the performance test again. However, in the time between the two performance test runs, the transaction name has been changed to TRX_01_BuyBook.

As a result of this inconsistent naming, you cannot obtain any trending information for this measurement, as Performance Center cannot recognize that the two transactions are actually the same, and compare them for trending purposes.

To overcome this problem, you map the two measurements (BuyBook and TRX_01_BuyBook) to a new third measurement which you create, for example Buy_Book_mapped. You add this new user-defined measurement to the trend report. Performance Center can then compare two instances of the Buy_Book_mapped transaction and give you meaningful trending information.

You can give the new transaction the same name as one of the current transactions. Additionally, you can configure the mapping so that all future instances of the transaction are automatically mapped to the new transaction name.

• Inconsistent monitor name

You want to compare your application's performance when it runs on different operating systems or when it runs on different Web/application servers.

You run the performance test once on a Windows platform, and then again on a Linux platform. You then want to compare the CPU utilization between the two runs. However, each platform provides a different name for this measurement. For example, % Processor Time (Processor_Total) in Windows, and CPU Utilization in Linux.

Performance Center cannot successfully obtain trending information for this measurement because the measurement names may be different.

To overcome this problem, you map the two measurements (% Processor Time (Processor_Total) and CPU Utilization) to a third measurement which you create, for example CPU_mapped. You then add this new user-defined measurement to the trend report. Performance Center can then compare the two instances of the CPU_mapped transaction and give you meaningful trending information.

You can give the new monitor the same name as one of the current monitors. Additionally, you can configure the mapping so that all future instances of the monitor are automatically mapped to the new monitor name.