Worst-Case Execution Time (WCET) analysis derives upper bounds for the execution times of programs. Such bounds are crucial when designing and verifying real-time systems. Static WCET analysis derives safe upper bounds. For complex hardware architectures the hardware modelling is still a challenge, leading to long analysis times and a risk of large WCET overestimation. Therefore, hybrid WCET analysis methods have appeared, where measurements are used to augment or replace the detailed low-level static WCET analysis. These methods do not in general yield a safe WCET estimate, but can still be appropriate in soft real-time systems where such WCET estimates are not crucial.In this paper we make two contributions. First, we develop a hybrid WCET analysis method, which uses regression to identify parameters in the common linear Implicit Path Enumeration Technique (IPET) model for WCET calculation.