HarnessML: Synthesis

Use after running several experiments, when you need to step back and connect the learnings. This is also the skill for deciding what to do next — and for recognizing when further experiments aren't adding understanding.

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Why Synthesis Matters

Individual experiments produce individual findings. Synthesis produces understanding. After 10 experiments, you should be able to say: "Here is what drives this target, here is where the model struggles, and here is why."

Without synthesis, you have a list of experiments. With synthesis, you have a theory.

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The Synthesis Questions

Periodically — every 5-10 experiments, or whenever you feel stuck — stop and answer:

What do we know?

Summarize the confirmed findings. What features matter and why? What model types work best and why? What relationships has the data revealed?

This is not a list of experiment results. It's a narrative: "The target is primarily driven by X and Y. Feature Z captures X well, but Y is only partially captured — the model struggles when Y interacts with W."

What have we ruled out?

Equally valuable. What hypotheses were tested and refuted? What strategies were exhausted? This prevents revisiting dead ends and focuses future work.

Where does the model still struggle?

Look at the diagnostic patterns across experiments:

• Are there consistent weak folds? What do they share?
• Are there subgroups with high error rates?
• Is there a ceiling you keep hitting?

This is where the next breakthrough will come from — not from tuning what works, but from understanding what doesn't.

What's the current theory?

State your best understanding of what drives the target. This theory should evolve with each experiment. If it hasn't changed in 5 experiments, either you're confirming it (good) or you're not learning (bad — try something different).

What are the highest-value next experiments?

Based on everything above, what would teach you the most? Not what would improve the metric the most — what would improve your understanding the most. Often these are the same thing, but not always.

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The Experiment Journal Review

experiments(action="journal")

Read through the journal with fresh eyes. Look for:

• Patterns across experiments: Did all feature-addition experiments help more than all hyperparameter experiments? That tells you where the signal lives.
• Contradictions: Did experiment 3 suggest X but experiment 7 suggest the opposite? What changed? Was it a confound?
• Diminishing returns: Are recent experiments producing smaller gains than early ones? That's natural, but at some point it means the model has extracted most of what the data offers.
• Unexplored directions: What hypotheses from early experiments were noted as "follow-up" but never pursued?

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When to Stop

There is no "done." But there is a point of diminishing returns where additional experiments aren't adding meaningful understanding.

Signals you're approaching diminishing returns:

• Last 3+ experiments produced <0.001 metric change
• Your theory of what drives the target hasn't been updated in 5+ experiments
• Diagnostic patterns are stable — the same folds are weak, the same subgroups are hard
• New features are highly correlated with existing ones (the novel signal space is exhausted)
• Meta-learner coefficients are stable across experiments

What to do at this point:

Don't declare victory. Instead, present the state of knowledge:

1. Current model performance with key metrics and per-fold stability
2. What drives the target — your best theory, with evidence
3. Where the model struggles — honest about remaining weaknesses
4. What would improve it further — if there were more data, different data, or more time, what would you investigate?
5. Confidence assessment — how robust is this model? Is it overfit to the training period? Would it generalize to new data?

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The Synthesis Template

## Project Synthesis — [Date or Experiment Count]

### State of Knowledge
[Narrative of what drives the target and how the model captures it]

### Key Findings
[3-5 most important confirmed learnings, with evidence]

### Ruled Out
[Strategies and hypotheses that were tested and refuted]

### Remaining Weaknesses
[Where the model struggles and current best understanding of why]

### Model Composition
[What's in the ensemble, why each model is there, diversity assessment]

### Recommended Next Steps
[Ranked by expected learning value, not expected metric gain]

### Open Questions
[What you'd investigate with more time/data]

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Persist the Synthesis

Write the synthesis to the notebook so it carries across sessions:

notebook(action="write", type="theory", content="[current understanding of what drives the target]")
notebook(action="write", type="plan", content="[ranked next steps with reasoning]")

The

theory

entry supersedes the previous one — only the latest is shown in the summary. The

plan

entry becomes the starting point for the next session.

If you ruled out strategies, record those too:

notebook(action="write", type="decision", content="Abandoned [strategy] because [structural reason]")

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Connecting to Other Skills

Synthesis naturally triggers other skills:

• "The model struggles with segment X" →

  diagnosis

  to understand the errors,

  domain-research

  to generate hypotheses about X's behavior
• "We haven't tried model family Y" →

  model-diversity

  to evaluate whether it would add value
• "Feature area Z is under-explored" →

  feature-engineering

  with domain-driven hypotheses
• "Results are contradictory" →

  eda

  to re-examine the data in light of new understanding
• "I don't know what to try next" →

  domain-research

  to bring in external knowledge

Synthesis is the hub. Everything flows through understanding.