# Measures Check

Use these questions to prepare for the Kanoa Ops certification exam.

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<summary>Why does a measure store a summary instead of every raw sample?</summary>

High-frequency signals would generate millions of rows a day. A measure collects values over a window and writes one record with the mean, standard deviation, min, max, sample count and control limits, keeping the shape of the signal at a fraction of the storage. See [Measures](../../../kanoa-ops/measures.md).

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<summary>What are the two ways to define a measure's collection window?</summary>

By sample count (`maxSampleCount`) or by duration (`maxSampleDurationSecs`). A record is written when the window completes. See [Configure a Measure](../10-measures/1-configure-measures.md).

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<summary>What does enabling the Tag option on a measure create, and what must you connect?</summary>

A measure UDT instance in a `measures` folder under the asset's tags. Its `value` tag must be pointed at the source signal, and `enabled` set to `true`, before it starts collecting. See [Configure a Measure](../10-measures/1-configure-measures.md).

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<summary>Why does each measure record keep its sample count?</summary>

So later roll-ups across many windows, lines or sites can be weighted properly, instead of treating every window as one data point. See [Measures](../../../kanoa-ops/measures.md).

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<summary>What does a Large Gap in the Gap Analysis table usually mean?</summary>

Collection stopped or stalled: the tag was disabled, the source disconnected, or the gateway restarted. Check gaps before using the data for reporting. See [Analyze Measures and SPC](../10-measures/2-analyze-measures.md).

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<summary>What is needed to run SPC checks on a measure?</summary>

A Quality license, a quality attribute with tolerances linked to the measure, and `spcCheckEnabled` set to `true` on the measure tag. Violations set `outOfControl`, are listed in `ruleViolations`, and appear in the notifications widget. See [Analyze Measures and SPC](../10-measures/2-analyze-measures.md).

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