Blog, page 6

Field notes — one a day since May 2023. Things we shipped, things we broke, and numbers we went and checked.

2026 2025 2024 2023

1,200 posts. , or browse by year and tag below.

Apr 10, 2026

What "free" means here, precisely

Writing is CC BY 4.0. Datasets are CC0 — public domain, no attribution required, because they are synthetic and there is nothing to attribute. Code is

practice Data platform

Apr 9, 2026

The interval is the result

A point estimate of 10.3%, the interval around it, and what a tighter one would cost.

statistics Statistics

Apr 8, 2026

The chart rules behind Data Explorer

A fast table with per-column profiling: type inference, missing counts, distinct values, quantiles, and a sparkline distribution for every numeric column.

visualization Visualization

Apr 7, 2026

Flight delays, in one paragraph

Eight thousand domestic flights across six carriers and fourteen airports, with departure and arrival delay, cancellations, and a delay distribution that is long-tailed rather than normal..

quality practice Data quality

Apr 6, 2026

Course note — The patterns that keep coming up

A short catalogue of the query shapes that appear in real work over and over — cohorts, funnels, running totals, deduplication, pivots — with the version that actually works.

practice Analytics engineering

Apr 5, 2026

The category that eats the chart

3 values with 54.7% concentrated in one of them, and what that does to every chart downstream.

visualization practice Visualization

Apr 4, 2026

Pattern: Conditional pivot

Reshape long to wide with CASE inside an aggregate. Portable, readable, and it works everywhere.

sql Analytics engineering

Apr 3, 2026

The time coverage of saas-subscriptions

624 populated days across a 1,045-day span, and what that allows you to compute.

engineering quality Data engineering

Apr 2, 2026

Comparing a period to the one before it

period comparison against a real schema, and the clause in it that is doing the work.

sql Analytics engineering

Apr 1, 2026

Zero is not a small number

89.7% of one column is exactly zero. Whether that is a measurement or an absence changes every aggregate built on it.

quality statistics Data quality

Mar 31, 2026

The total came out 34.6× too high

34.6 rows per key on one table, and the totals that quietly inherit the multiplier.

sql quality Analytics engineering

Mar 30, 2026

Counting rows is not testing grain

3,779 distinct combinations against 4,000 rows, and what that answers about the table's key.

sql quality warehousing Analytics engineering

Mar 29, 2026

The overall average hides 6 different numbers

Real group means for one column across 6 segments, and what the pooled average conceals.

statistics practice Analytics practice

Mar 28, 2026

The average sessions is not the typical one

A 27.1% gap between mean and median on one column, and which of the two answers the question you were asked.

statistics practice Statistics

Mar 27, 2026

1.6% of pm25 is missing

85 blanks in one column, and the one-line check that decides whether dropping them is free.

quality statistics Statistics

Mar 26, 2026

Five minutes with cancelled in flight-delays

8,000 rows, 2 distinct values, and the one fact about cancelled that changes how you query it.

quality practice Data quality

Mar 25, 2026

Charting units by country

Two facts about the columns decide the chart. Cardinality and skew, in that order.

visualization Visualization

Mar 24, 2026

The arithmetic behind a cheap query

Column pruning times partition pruning on a real schema, and the function call that quietly defeats both.

performance warehousing Data platform

Mar 23, 2026

Before you clip the outliers

A max 1.1× the 99th percentile, and the three different things that could be.

statistics quality Statistics

Mar 22, 2026

r = 0.50 between temp_c and solar_mw

A measured correlation of 0.50 between two columns, what its square says, and the diagnostic that costs one line.

statistics Statistics

Mar 21, 2026

The interval is the result

A point estimate of 9.7%, the interval around it, and what a tighter one would cost.

statistics Statistics

Mar 20, 2026

Correction: Forward-chaining split

The page claimed more than it should have. What it says now, and where the pattern stops applying.

quality practice Machine learning

Mar 19, 2026

Why SQL Playground works the way it does

Write real SELECT queries against any dataset here and get results instantly — joins, aggregates, GROUP BY, HAVING and ORDER BY, with plain errors for the things it does not support.

visualization Visualization

Mar 18, 2026

What is deliberately wrong with saas-subscriptions

Twenty-five hundred B2B subscription accounts with signup dates, plans, seat counts, MRR and churn dates.

quality practice Data quality