Blog, page 7

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.

Mar 17, 2026

Site search without a server, in about 80 lines

A JSON index, term scoring, and why we did not reach for a search library.

engineering Visualization

Mar 16, 2026

retail-orders.category: 6 values, 59.5% in the top three

6 values with 24.2% concentrated in one of them, and what that does to every chart downstream.

visualization practice Visualization

Mar 15, 2026

Course note — A working setup that will not embarrass you

The minimum professional toolchain — version control, environments, notebooks used correctly, and the habits that make work reproducible six months later by someone who is not you.

practice Data engineering

Mar 14, 2026

Merge with a recency guard, in short

Upsert on a natural key, and refuse to let an out-of-order replay overwrite good data with stale data.

pipelines Data engineering

Mar 13, 2026

pickup_at runs 2023-01-01 to 2023-12-31

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

engineering quality Data engineering

Mar 12, 2026

Keeping one row per key in retail-orders

deduplicate by rank against a real schema, and the clause in it that is doing the work.

sql Analytics engineering

Mar 11, 2026

Is resolved_at + channel 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

Mar 10, 2026

Check the blanks before you drop them

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

quality statistics Statistics

Mar 9, 2026

gdp_per_capita_usd: mean 26,959, median 20,761

A 29.9% gap between mean and median on one column, and which of the two answers the question you were asked.

statistics practice Statistics

Mar 8, 2026

What salary_max_usd actually contains in data-job-postings

3,500 rows, 554 distinct values, and the one fact about salary_max_usd that changes how you query it.

quality practice Data quality

Mar 7, 2026

Reading 3 columns instead of 10

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

performance warehousing Data platform

Mar 6, 2026

Before you clip the outliers

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

statistics quality Statistics

Mar 5, 2026

first_response_min by channel: a 11% spread

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

statistics practice Analytics practice

Mar 4, 2026

A essentially none correlation, and what it is not

A measured correlation of 0.08 between two columns, what its square says, and the diagnostic that costs one line.

statistics Statistics

Mar 3, 2026

Correction: The grain uniqueness test

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

quality practice Data quality

Mar 2, 2026

Charting salary_max_usd by seniority

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

visualization Visualization

Mar 1, 2026

The interval is the result

A point estimate of 38.0%, the interval around it, and what a tighter one would cost.

statistics Statistics

Feb 28, 2026

A five-minute use for Distribution Lab

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

Feb 27, 2026

What is deliberately wrong with sensor-telemetry

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

Feb 26, 2026