Blog, page 18

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.

Jun 26, 2025

The average seats is not the typical one

A 533.8% gap between mean and median on one column, and which of the two answers the question you were asked.

statistics practice Statistics

Jun 25, 2025

What co2_tonnes_per_capita actually contains in world-indicators

720 rows, 686 distinct values, and the one fact about co2_tonnes_per_capita that changes how you query it.

quality practice Data quality

Jun 24, 2025

We turned off 80% of our alerts and caught more incidents

Alert fatigue is a data quality problem, and the fix is deletion.

production practice Data platform

Jun 23, 2025

Before you clip the outliers

A max 1.2× the 99th percentile, and the three different things that could be.

statistics quality Statistics

Jun 22, 2025

salary_max_usd by seniority: a 162% spread

Real group means for one column across 5 segments, and what the pooled average conceals.

statistics practice Analytics practice

Jun 21, 2025

A essentially none correlation, and what it is not

A measured correlation of 0.11 between two columns, what its square says, and the diagnostic that costs one line.

statistics Statistics

Jun 20, 2025

Charting revenue_usd by variant

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

visualization Visualization

Jun 19, 2025

The chart rules behind Experiment Calculator

Sample size before you launch, and a confidence interval after you stop. Both directions, with the assumptions written on the screen rather than buried in a footnote.

visualization Visualization

Jun 18, 2025

Customer support tickets, in one paragraph

Four thousand support tickets with free-text subjects, priorities, queue routing, first-response and resolution times, and SLA breaches concentrated in exactly the places you would not guess..

quality practice Data quality

Jun 17, 2025

Cutting a lesson down — Causal claims when you cannot randomise

Half the interesting questions cannot be tested. Here is what is available instead, what each method assumes, and how to state a conclusion you can defend.

practice Experimentation

Jun 16, 2025

Fan-out join, in short

A join to a table with more than one row per key multiplies every measure on the other side. Nothing errors, and the number is plausible.

sql Analytics engineering

Jun 15, 2025

The category that eats the chart

14 values with 7.6% concentrated in one of them, and what that does to every chart downstream.

visualization practice Visualization

Jun 14, 2025

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

Jun 13, 2025

distance_km: mean 4.83, median 3.20

A 51.0% gap between mean and median on one column, and which of the two answers the question you were asked.

statistics practice Statistics

Jun 12, 2025

What pm25 actually contains in city-air-quality

5,480 rows, 480 distinct values, and the one fact about pm25 that changes how you query it.

quality practice Data quality

Jun 11, 2025

Is resolved_at + priority the grain of support-tickets?

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

sql quality warehousing Analytics engineering

Jun 10, 2025

The top of humidity_pct is 1.1× the 99th percentile

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

statistics quality Statistics

Jun 9, 2025

fare_usd by payment_type: a 4% spread

Real group means for one column across 3 segments, and what the pooled average conceals.

statistics practice Analytics practice

Jun 8, 2025

A strong correlation, and what it is not

A measured correlation of 0.80 between two columns, what its square says, and the diagnostic that costs one line.

statistics Statistics

Jun 7, 2025

Charting flight_no by carrier

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

visualization Visualization

Jun 6, 2025

The chart rules behind SQL Playground

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

Jun 5, 2025

An exercise on support-tickets

Four thousand support tickets with free-text subjects, priorities, queue routing, first-response and resolution times, and SLA breaches concentrated in exactly the places you would not guess..

quality practice Data quality

Jun 4, 2025

Cutting a lesson down — Describing a column without lying

The mean is the default summary and it is frequently the wrong one. Knowing when — and what to reach for instead — is most of applied statistics in this job.

practice Statistics

Jun 3, 2025

Counting rows is not testing grain

7,193 distinct combinations against 8,000 rows, and what that answers about the table's key.

sql quality warehousing Analytics engineering