Nested Experiments

Nested experiments are the DOE structures that organize factors in hierarchical levels when some variables are harder or slower to change than others - they preserve statistical power while making fab experimentation operationally feasible under real tool and schedule constraints.

What Is Nested experiments?

• Definition: Experimental design where one factor level exists inside another, such as runs nested within chamber or lot nested within tool.
• Typical Use: Split-plot and split-split-plot studies where temperature may change daily but gas flow can change per run.
• Statistical Model: Mixed-effects analysis separates between-group and within-group variability correctly.
• Output: Reliable estimates for main effects and interactions without violating practical run constraints.

Why Nested experiments Matters

• Operational Realism: Hard-to-change factors can be tested without unrealistic run sequencing.
• Data Integrity: Prevents incorrect ANOVA conclusions caused by ignoring hierarchical error structure.
• Cycle-Time Control: Reduces costly recipe changeovers while still extracting meaningful cause-effect insight.
• Scale-Up Value: Nested designs map better to real production logistics than idealized full randomization.
• Decision Confidence: Teams can quantify which variability source is tool-level versus run-level.

How It Is Used in Practice

• Hierarchy Planning: Classify each factor as hard-to-change or easy-to-change before matrix construction.
• Run Execution: Sequence experiments by whole-plot groups, then randomize sub-plot settings within each group.
• Model Fitting: Use mixed-model software to estimate effects and confidence intervals with correct error terms.

Nested experiments are the practical DOE framework for complex manufacturing realities - they deliver valid statistical conclusions without breaking fab execution constraints.