In my post, Test Developers Shouldn't Execute Tests, Antony Marcano asked if we actually need test developers or if developers would do. If the more traditional testing tasks are being done by one group and the automation by another, does it even make sense to have a test development role any more? The answer to this question is interesting enough that it warrants its own post. The answer I've come to is yes.
The skills employed by test developers are very similar to those utilized by developers. In fact, I've often argued that test developers are merely developers in the test organization. I stand by that description. There are some differences that we'll see below, but it is largely true. The vast majority of the time a test developer is working his craft, he'll be writing code which will look very similar to the code written by developers. If the product is a framework or platform, the code a test developer writes is the same code written by all developers utilizing the framework. The constraints on the code are different. The level of polish is usually not as high. The execution path for test code is very constrained and so the code need not be as robust in the rest of the path. High quality is, however, still very important. Tests cannot be flaky and often have long lifespans so correct behavior and maintainability are important items. These are the same sorts of things developers are required to produce in their code.
So why do we need this class of people? The skillset, while similar, is also different. Also, it is important to have a role that is set aside just for this task.
Test developers must think differently than developers. Whereas a developer must worry about the positive cases, a test developer focuses her efforts on the negative cases. She is more interested in the corner cases and "unexpected" input than on the primary method of operation. This is even more true today in the world of increasing use of unit tests. Test developers also must focus on end-to-end and integration testing. This is often the hardest to automate and requires a different approach than the typical unit tests take. Developers could write this code, but are not accustomed to doing so. Having a person who does this all the time will produce better refined solutions. In short, test developers will have a greater sense of where to look for bugs than someone who does not spend most of their day trying to break things.
There are particular methodologies specific to test development. Developers will be unfamiliar with these. I'm thinking of model-based testing, data-driven testing, equivalency classes, pairwise testing, etc. Being adept at each of these is a skill that requires development. A person dedicated to test development will learn them. Someone recruited for only a short time to write some tests will not.
Additionally, there is a benefit to having a dedicated test development corps beyond just the specific skills. If everyone is thrown into a common bucket, testing tends to get underfunded. The money is not made directly by testing. In fact, you can usually skimp on testing for a while and get away with it. Over time though, a product will quickly degrade if it is not actively testing. Having a sandboxed team responsible for testing ensures that proper time is spent testing. No one can steal cycles from the test development team to implement one more feature.
It's also important to have a group that is not invested in the product the same way that dev is. A good tester will be proud to hold up the product if it deserves it. Developers will, subconsciously, be rooting for the product to succeed and may overlook things that are wrong. They'll also approach the product with assumptions that someone involved in the creation will not. What is obvious to everyone who wrote the product may not be obvious to anyone else.