Blog, page 2

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

2026 2025 2024 2023

1,244 posts. , or browse by year and tag below.

Aug 28, 2026

Correction: Counting your tests

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

quality practice Experimentation

Aug 27, 2026

Should I use dbt?

If you have more than a handful of SQL models and more than one person editing them, yes — and mostly for reasons that have nothing to do with the too

warehousing engineering Analytics engineering

Aug 26, 2026

Search is a JSON file

Site search is a static JSON index fetched on first use and scored in the page. No search service, no API key, no query leaving your browser.

engineering performance Data platform

Aug 25, 2026

10.3%, plus or minus what?

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

statistics Statistics

Aug 24, 2026

The chart rules behind Regression Lab

Fit an ordinary least squares line to any pair of columns and get the diagnostics that decide whether the fit means anything: residual plot, R², standard errors, and leverage.

visualization Visualization

Aug 23, 2026

An exercise on movie-ratings

Nine thousand user ratings over 260 films, with genres, release years, and the long-tail popularity distribution that makes naive recommenders look better than they are..

quality practice Data quality

Aug 22, 2026

Reader question — 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

Aug 21, 2026

Pattern: Cohort retention triangle

Group by when they started, measure how many remain N periods later — and remember that the bottom-right corner is young, not good.

statistics Analytics practice

Aug 20, 2026

The time coverage of ride-hail-trips

365 populated days across a 365-day span, and what that allows you to compute.

engineering quality Data engineering

Aug 19, 2026

world-indicators.country: 30 values, 10.0% in the top three

30 values with 3.3% concentrated in one of them, and what that does to every chart downstream.

visualization practice Visualization

Aug 18, 2026

We added Anscombe's quartet to the correlation explorer

Four datasets with identical statistics and completely different shapes, now one click away.

visualization statistics Visualization

Aug 17, 2026

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 16, 2026

customer_id is not unique, and the join knows

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

sql quality Analytics engineering

Aug 15, 2026

Is resolved_at + ticket_id the grain of support-tickets?

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

sql quality warehousing Analytics engineering

Aug 14, 2026

Reading country before trusting it in data-job-postings

3,500 rows, 7 distinct values, and the one fact about country that changes how you query it.

quality practice Data quality

Aug 13, 2026

Check the blanks before you drop them

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

quality statistics Statistics

Aug 12, 2026

1,146 zeros in tip_usd, and what they mean

14.3% of one column is exactly zero. Whether that is a measurement or an absence changes every aggregate built on it.

quality statistics Data quality

Aug 11, 2026

sessions: mean 3, median 2

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

Aug 10, 2026

Before you clip the outliers

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

statistics quality Statistics

Aug 9, 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

Aug 8, 2026

The overall average hides 8 different numbers

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

statistics practice Analytics practice

Aug 7, 2026

A essentially none correlation, and what it is not

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

statistics Statistics

Aug 6, 2026

What 3,500 job postings say about city

One column of a postings dataset, and what the shape of it means for anyone reading listings.

career practice Analytics practice

Aug 5, 2026

Correction: As-of feature computation

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

quality practice Machine learning