Creating an Executable Specification from existing manual test scripts might seem as a logical thing to do when starting out with Specification by Example and Agile Acceptance Testing. Such scripts already describe what the system does, and the testers are running them anyway, so automation will surely help. Not really — this is in fact one of the most common failure patterns.
The problem is that manual and automated checks are affected by a completely different set of constraints. With manual testing, the time spent preparing the context is often a key bottleneck. With automated testing, people spend most time on understanding what is wrong when a test fails.
For example, to prepare for a manual test that checks user account management rules, you might have to log on to an administrative application, create a user, log on as that new user to the client application, and change the password after first use. To avoid doing this several times during the test, manual scripts often reuse the context. So you would create the user once, block that account and verify that the user cannot log on, reset the password to verify that it is re-enabled, then set some user preferences and verify they change the home page correctly. This approach helps a tester run through the script quicker.
With automated testing, time spent on setting up the user is no longer a problem. Automated tests generally go through many more cases than manual tests, and when they run correctly nobody is really looking at them. Once a test fails, someone has to go in and understand what went wrong. If a test is described as a sequence of interdependent steps, it will be very hard to understand what exactly caused the problem, because the context changes throughout the script. The fact that a single script is checking ten different things also makes it more probable that the test will fail because it is affected by lots of different areas of code. In the previous example with user account management, if the password reset function stops working, we won’t be able to set the user preferences correctly. If we had ten different, smaller, focused and independent tests instead of one big script, a bug in the password reset function won’t affect the test results for user preferences. That makes tests more resilient to change and reduces the cost of maintenance. It also helps us pin-point the problems quicker.
Instead of plainly automating manual test scripts, think about what the script is testing and describe that with a group of independent, focused tests. This will significantly reduce the automation overhead and maintenance costs.