# Null explanation / detection in Ask data: New in Tableau 2021.3

> This is content from just-tim, the data-and-analytics channel by Tim Ngwena (formerly 'Tableau Tim'). Tim has 12+ years of hands-on BI experience and covers Tableau most of all, plus Power BI, Looker, Hex, SQL and data modelling, the analytics industry, and the craft of doing the job — always tool-agnostic and honest about the trade-offs.

- **Author:** Tim Ngwena (just-tim, https://just-tim.com/about)
- **Published:** 2021-09-13
- **Format:** Video · 14 min watch · transcript available
- **Topics:** Data visualisation, Analytics
- **Tools:** Tableau (ai, calculated fields)
- **Canonical:** https://just-tim.com/posts/null-explanation-detection-in-ask-data-new-in-tableau-2021-3
- **Watch:** https://www.youtube.com/watch?v=HHe4kHSB9HI

I explore the new null value explanation type added to Explain Data (Ask Data) in Tableau 2021.3. Using a deliberately constructed null-heavy dataset, I try to trigger the feature and run into inconsistent behaviour, spending most of the video troubleshooting how the algorithm actually decides what counts as a higher-than-expected amount of nulls.

## Key takeaways

- Explain Data only works on aggregated marks, so nulls won't be detected on a scatter plot where individual data points render null and disappear from the view.
- The new 'percentage of nulls' explanation type can be enabled and filtered to via the Explain Data configuration wheel under explanation types.
- Visual grouping and Tableau's automatic 'other' grouping can break Explain Data, so manually grouping records works more reliably.
- Double-clicking the generated field (or control-dragging it into the calculation window) reveals the underlying SUM(IF null then 1 else 0) calculation Tableau builds on the fly.
- Tableau's documentation describes a 'higher than expected amount of missing data' but never defines the threshold, so a dataset deliberately full of nulls may normalise and fail to trigger the alert.

## Chapters

- 0:14 Building a null-heavy test dataset
- 1:03 Connecting the CSV in Tableau
- 2:13 Why scatter plots don't trigger Explain Data
- 3:04 Visual grouping attempts and failures
- 4:55 Enabling the percentage of nulls type
- 7:09 Manual grouping finally triggers it
- 9:38 Inspecting the underlying calculation
- 10:18 Testing lower null percentages
- 11:49 Checking the documentation for a threshold

Watch the full video, read the transcript and use chapter deep-links on the page: https://just-tim.com/posts/null-explanation-detection-in-ask-data-new-in-tableau-2021-3

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just-tim — Data and analytics, with a point of view. · https://www.youtube.com/channel/UC7HYxRWmaNlJux-X7rNLZyw · https://twitter.com/TableauTim · https://www.linkedin.com/in/timngwena