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Week 23 · Build: Unseen conditions test the claim

Session 2 of 4 · Test under pressure · Phase 6

Plan about 15 minutes for explanation, 30 minutes for practical work and 10–15 minutes for documentation. A longer build may continue into the next session: stop safely, commit the current state and record the next check. Desktop simulations count as software evidence; label them clearly and record physical validation separately.

Engineering challenge

How reliable is your system within its claimed conditions, including the failures? This session focuses on unseen conditions test the claim.

Before you start

The previous week’s recorded baseline and Week 22, Improve. For later sessions this week, retain the preceding session’s files and predictions.

Equipment: Desktop Python, editor, paper and ruler; for physical work, the configured 3pi+ 2040, clear floor mat and hardware checklist. Week 3 additionally uses the separate low-voltage LED circuit described in its procedure.

For any motion, verify the stop button, short time limit and clear floor area first. Keep the wheels raised for a new device program until its commands and stop behaviour are checked. A hazard or uncertain input is a reason to stop and document, not to force the trial to finish.

Theory and mathematics

Unseen conditions test the claim

A held-out condition should remain inside the declared operating envelope, but differ from the tuning cases—for example a different start angle or another surveyed route. Testing outside the envelope explores robustness and must be labelled separately. Change one factor when diagnosing causes; combine realistic conditions when evaluating the whole system. Randomize trial order when possible to reduce battery/practice confounding. Prepare the logging and acceptance rules before starting so failures are captured consistently.

Worked example — illustrative values

In ten fictional attempts, eight arrive and two stop on hazards. Navigation success is 8/10 = 80%; safe stopping on the two hazard cases is a separate outcome. If eight arrived errors total 0.48 m, their mean is 0.06 m, conditional on arrival. It is misleading to report only that mean and hide the two failed missions.

Write the calculation in your notebook before running code. State which values you measured, which you assumed and which the program calculates. A correct numerical calculation cannot rescue an incorrect physical assumption.

Run and explain the model

The following is desktop Python, not a ready-to-run motor program. Download this week’s example, save it in your student repository and run python3 code/w23.py from the repository root. The same small model is reused across the week so you can learn it, build with it, test it and revise it.

# Desktop Python teaching example. Numerical inputs are illustrative.
from statistics import mean
trials = [
    {"arrived": True, "error_m": 0.04, "reason": "arrived"},
    {"arrived": True, "error_m": 0.08, "reason": "arrived"},
    {"arrived": False, "error_m": None, "reason": "hazard"},
]
errors = [t["error_m"] for t in trials if t["arrived"]]
print("attempts", len(trials), "arrivals", len(errors))
print("arrival_fraction", len(errors) / len(trials))
print("mean_error_given_arrival_m", mean(errors) if errors else None)

Run the example on desktop Python before adapting it. Change one valid input and check the result; keep device-only calls in a separate adapter. If an exception appears, read its final line, identify the input or assumption that caused it and make the smallest explained correction. Do not delete validation merely to obtain output.

Understanding the model and its limits

The arrivals list filters successful outcomes for the conditional endpoint metric, but the denominator of arrival_fraction remains every attempt. This preserves the stopped failure. Missing endpoint measurement is represented by None rather than zero. In the three-row example, arrivals are 2/3 and mean error conditional on arrival is 0.06 m. A real report also groups conditions and versions; one overall fraction can hide a systematically failing route. A causal diagnosis needs the first divergent log sample or a controlled reproduction, not just a final failure label.

Practical instructions

  1. Prepare course markings, start fixtures, logging and an observer checklist.
  2. Verify one rehearsal record without including it in validation.
  3. Alternate/randomize test conditions and save an empty results table before running.
  4. Run the first held-out batch without changing code or settings midway.

Experiment

Test the recording process on normal arrival and an injected stop; both must produce complete outcome records.

Before testing, record your prediction, changed factor, measured response, fixed conditions and stopping rule. Save every attempted run, including failures, with a condition and source version. If hardware is unavailable, use an explicitly labelled synthetic/replay dataset and list the physical question it cannot answer. Do not invent completed trials.

Deliverable

A frozen-run recording workflow that retains failures and measurement status.

Save notebook/w23-s2.md, the relevant code revision, raw CSV or test-case records, and one labelled diagram/plot/table. Link the files relatively from your notebook. Use the entry template and report guide.

Completion criteria

A documented failed prediction can meet the learning criteria. A missing physical trial must remain marked untested; software success alone does not validate the robot.

Reading and video

Reflection and next step

Which assumption most affected your result? Point to one observation that supports your explanation and one alternative explanation the evidence has not ruled out. Write a specific next test with a changed factor and measurable outcome, then proceed through the week’s Learn → Build → Experiment → Improve cycle.

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