Blog, page 17

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.

Jul 20, 2025

The top of mrr_usd is 1.6× the 99th percentile

A max 1.6× the 99th percentile, and the three different things that could be.

statistics quality Statistics

Jul 19, 2025

distance_km by payment_type: a 3% spread

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

statistics practice Analytics practice

Jul 18, 2025

r = 0.72 between load_mw and price_eur_mwh

A measured correlation of 0.72 between two columns, what its square says, and the diagnostic that costs one line.

statistics Statistics

Jul 17, 2025

Charting year by region

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

visualization Visualization

Jul 16, 2025

Why Experiment Calculator works the way it does

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

Jul 15, 2025

City air quality, in one paragraph

Daily pollutant readings for four cities over nearly four years, with strong seasonality, weekday effects, wildfire episodes, and genuine sensor outages..

quality practice Data quality

Jul 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

Jul 13, 2025

Course note — The anatomy of a silent failure

A walk through a real class of incident — the kind where every job is green, every test passes, and the number has been wrong for six weeks.

practice Data quality

Jul 12, 2025

Contract check at the boundary, in short

Validate the shape of incoming data where it enters your system — fail loudly on structure, warn on distribution.

quality pipelines Data quality

Jul 11, 2025

city-air-quality.city: 4 values, 75.0% in the top three

4 values with 25.0% concentrated in one of them, and what that does to every chart downstream.

visualization practice Visualization

Jul 10, 2025

Counting rows is not testing grain

260 distinct combinations against 9,000 rows, and what that answers about the table's key.

sql quality warehousing Analytics engineering

Jul 9, 2025

dep_delay_min: mean 10, median 5

A 98.7% gap between mean and median on one column, and which of the two answers the question you were asked.

statistics practice Statistics

Jul 8, 2025

What signed_up_on actually contains in saas-subscriptions

2,500 rows, 947 distinct values, and the one fact about signed_up_on that changes how you query it.

quality practice Data quality

Jul 7, 2025

Before you clip the outliers

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

statistics quality Statistics

Jul 6, 2025

The overall average hides 4 different numbers

Real group means for one column across 4 segments, and what the pooled average conceals.

statistics practice Analytics practice

Jul 5, 2025

r = -0.36 between pm10 and o3_ppb

A measured correlation of -0.36 between two columns, what its square says, and the diagnostic that costs one line.

statistics Statistics

Jul 4, 2025

The category that eats the chart

6 values with 26.4% concentrated in one of them, and what that does to every chart downstream.

visualization practice Visualization

Jul 3, 2025

Charting year by country

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

visualization Visualization

Jul 2, 2025

What Correlation Explorer will not do

A diverging heatmap of every numeric pair in a dataset, with a click-through scatter so you can check whether the number is telling the truth.

visualization Visualization

Jul 1, 2025

What is deliberately wrong with 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 30, 2025