Definition
The Initial Transient Phase (or Warm-up Period) is the beginning portion of a simulation run during which the system’s state variables are moving from their initial conditions (often “empty and idle”) toward a stable operating state (steady state).
Why It Matters
Starting a system (like a new factory or a computer simulation) often involves a “messy” beginning that doesn’t reflect how it will actually run in the long term. If you make decisions based on this initial “warm-up” data, you will get a distorted view of reality. Recognizing the transient phase allows you to “wait for the dust to settle” before trusting your statistics or your strategy.
Core Concepts
- Bias: Including data from the transient phase in the final analysis of a nonterminating system will bias the results (e.g., underestimating queue lengths or utilization).
- Determining Length:
- Visual Inspection: Plotting a moving average of a key MOP and identifying where the curve flattens.
- Welch’s Method: A statistical approach to identify the point where variance stabilizes.
- Warm-up Specification: Most simulation software (ARENA, AutoMod) allows the user to specify a “Warm-up Time” after which the statistical accumulators are reset to zero.