Page 6 of 8~104 min topic

Data: tables and simple plots

Instrument the study table and plot

Page 6 adds signals that distinguish bad input from component failure in the study-hours table plus hours-vs-pass plot.

~13 min this pageTesting and observability

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Emit stage signals

Instrument the study-hours table plus hours-vs-pass plot so a run records enough structure to debug offline: counts, latency if relevant, pass/fail of mean hours, max hours, and plot file exists with xlabel/ylabel set, and a stable stage name. Redact secrets and raw credentials from every event.

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Emit and assert

import json
print(json.dumps({'rows':4,'mean_hours':5.25,'max_hours':12,'plot':'hours_vs_pass.png'}))

Expected evidence: summary metrics JSON. Prefer JSON or structured text you can grep in CI over prose logs for data-tables-and-plots.

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Lock signals with a regression test

Turn one historical failure—especially wrong column names—into a test that fails if the signal disappears for the study table and plot. Observability without a failing test is optional decoration; observability with a test is part of the data-tables-and-plots artifact.

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Lab notebook: signal schema

Draft a three-field event for the study table and plot: stage, ok, and one domain field derived from mean hours, max hours, and plot file exists with xlabel/ylabel set. Add fixture_id or docs_version when content can change. Explicitly list fields that must never appear (tokens, passwords, raw prompts) because publishing learner names from the CSV in a shared plot title is in scope for this lab.

Wire one assertion that fails if the study table and plot event is missing after a run. Observability that cannot fail a test will not survive contact with a busy data-tables-and-plots repository.

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Worked judgment

Imagine a teammate opens only your event stream after a bad deploy. Could they tell whether four-row study.csv with hours and passed columns was wrong, whether wrong column names, or a plot that saves without axis labels returned, or whether publishing learner names from the CSV in a shared plot title slipped through? If not, rename fields until those three stories are distinguishable.

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Why this stage matters for the study table and plot

At the testing and observability stage for data-tables-and-plots, the job is narrower than finishing a product demo. You are creating one progressive evidence piece about four-row study.csv with hours and passed columns that later pages inherit without redefining success. Keep that fixture small enough to inspect by hand, keep outputs copy-pasteable as text, and refuse to narrate this baseline as if it were a production SLA: hand-computed mean of the four-row fixture.

For this page specifically, success looks like a structured event schema locked by a test while still centering the user decision to inspect typical hours, an extreme value, and the hours/pass relationship. If you cannot point to a file, command, or assertion that proves that for the study table and plot, stay on this page instead of advancing.

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Before you start

Why this matters

Write the single log line or metric event that would tell you whether a bad result came from input vs implementation for the study table and plot. If your line could not tell them apart, redesign it before coding.

In the wild

See how this idea shows up as a product and a company — then come back to the lesson. Skills transfer across vendors.

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Page assessment

Answer from memory. Completion is saved from this evidence, not from opening the next page.

1. Can input faults be distinguished from component faults in the event?
2. Are secrets redacted from logs?
3. Is there a test that fails if the signal vanishes?
4. Does the event still reference the decision: inspect typical hours, an extreme value, and the hours/pass relationship?

All responses are required.