Blog, page 15

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.

Sep 6, 2025

saas-subscriptions.region: 4 values, 93.3% in the top three

4 values with 46.5% concentrated in one of them, and what that does to every chart downstream.

visualization practice Visualization

Sep 5, 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

Sep 4, 2025

Is user_id + movie_id the grain of movie-ratings?

8,291 distinct combinations against 9,000 rows, and what that answers about the table's key.

sql quality warehousing Analytics engineering

Sep 3, 2025

The average co2 tonnes per capita is not the typical one

A 31.1% gap between mean and median on one column, and which of the two answers the question you were asked.

statistics practice Statistics

Sep 2, 2025

The top of o3_ppb is 1.2× the 99th percentile

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

statistics quality Statistics

Sep 1, 2025

Five minutes with revenue_usd in retail-orders

7,000 rows, 5,719 distinct values, and the one fact about revenue_usd that changes how you query it.

quality practice Data quality

Aug 31, 2025

r = -0.00 between vibration_mm_s and rpm

A measured correlation of -0.00 between two columns, what its square says, and the diagnostic that costs one line.

statistics Statistics

Aug 30, 2025

The overall average hides 3 different numbers

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

statistics practice Analytics practice

Aug 29, 2025

Charting no2_ppb by city

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

visualization Visualization

Aug 28, 2025

Why Distribution Lab works the way it does

Sample from seven distributions, watch the central limit theorem happen at whatever sample size you choose, and see exactly when the mean stops being a useful summary.

visualization Visualization

Aug 27, 2025

Factory sensor telemetry, in one paragraph

Eight thousand readings from twelve machines across three sites, at ten-minute intervals, with drift, seasonal temperature, injected anomalies, and gaps where a gateway dropped offline..

quality practice Data quality

Aug 26, 2025

No runbook, no page

A rule that halved our overnight escalations.

production practice Data platform

Aug 25, 2025

Cutting a lesson down — Rows, grain, and the shape of a dataset

Nearly every wrong number in a career traces back to someone not knowing what one row of their table represents.

practice Analytics practice

Aug 24, 2025

Counting your tests, in short

Twenty segments at α = 0.05 gives you a one-in-two chance of a false finding. Correct for it, or at minimum say how many you ran.

statistics experiments Experimentation

Aug 23, 2025

rated_at runs 2023-01-01 to 2025-06-19

901 populated days across a 901-day span, and what that allows you to compute.

engineering quality Data engineering

Aug 22, 2025

The category that eats the chart

6 values with 21.9% concentrated in one of them, and what that does to every chart downstream.

visualization practice Visualization

Aug 21, 2025

The average surge multiplier is not the typical one

A 35.0% gap between mean and median on one column, and which of the two answers the question you were asked.

statistics practice Statistics

Aug 20, 2025

A running total that does not lie about ties

running total against a real schema, and the clause in it that is doing the work.

sql Analytics engineering

Aug 19, 2025

Counting rows is not testing grain

2,500 distinct combinations against 2,500 rows, and what that answers about the table's key.

sql quality warehousing Analytics engineering

Aug 18, 2025

What arr_delay_min actually contains in flight-delays

8,000 rows, 261 distinct values, and the one fact about arr_delay_min that changes how you query it.

quality practice Data quality

Aug 17, 2025

The top of duration_min is 1.3× the 99th percentile

A max 1.3× the 99th percentile, and the three different things that could be.

statistics quality Statistics

Aug 16, 2025

discount_pct by country: a 14% spread

Real group means for one column across 8 segments, and what the pooled average conceals.

statistics practice Analytics practice

Aug 15, 2025

r = 0.75 between life_expectancy and co2_tonnes_per_capita

A measured correlation of 0.75 between two columns, what its square says, and the diagnostic that costs one line.

statistics Statistics

Aug 14, 2025

Charting rating by genre

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

visualization Visualization