Economic perspectives in test automation: balancing automated and manual testing with opportunity cost

Testing is a major cost factor in software development. Test automation has been proposed as one solution to reduce these costs. Test automation tools promise to increase the number of tests they run and the frequency at which they run them. So why not automate every test? In this paper we discuss the question "When should a test be automated?" and the trade-off between automated and manual testing. We reveal problems in the overly simplistic cost models commonly used to make decisions about automating testing. We introduce an alternative model based on opportunity cost and present influencing factors on the decision of whether or not to invest in test automation. Our aim is to stimulate discussion about these factors as well as their influence on the benefits and costs of automated testing in order to support researchers and practitioners reflecting on proposed automation approaches.