Blog, page 21

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.

Apr 15, 2025

r = -0.09 between solar_mw and wind_mw

A measured correlation of -0.09 between two columns, what its square says, and the diagnostic that costs one line.

statistics Statistics

Apr 14, 2025

Charting vibration_mm_s by site

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

visualization Visualization

Apr 13, 2025

What Chart Builder will not do

Point it at any dataset, choose an x, a y, and a grouping, and get a chart that follows the rules — one axis, direct labels, a real legend, and a table view underneath.

visualization Visualization

Apr 12, 2025

An exercise on ride-hail-trips

Eight thousand city ride-hail trips with fares, distances, surge multipliers and tips.

quality practice Data quality

Apr 11, 2025

Course note — What a data team is actually for

The job is not producing numbers. It is reducing the number of decisions that get made badly, and almost everything else follows from that.

practice Analytics practice

Apr 10, 2025

A proportion with an interval, in short

A percentage without a denominator is not a finding. The Wilson interval behaves sensibly at small n, where the normal approximation falls apart.

statistics experiments Statistics

Apr 9, 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

Apr 8, 2025

Counting rows is not testing grain

4,169 distinct combinations against 7,000 rows, and what that answers about the table's key.

sql quality warehousing Analytics engineering

Apr 7, 2025

The category that eats the chart

3 values with 92.4% concentrated in one of them, and what that does to every chart downstream.

visualization practice Visualization

Apr 6, 2025

The overall average hides 14 different numbers

Real group means for one column across 14 segments, and what the pooled average conceals.

statistics practice Analytics practice

Apr 5, 2025

Five minutes with priority in support-tickets

4,000 rows, 4 distinct values, and the one fact about priority that changes how you query it.

quality practice Data quality

Apr 4, 2025

The average revenue usd is not the typical one

A 84.0% gap between mean and median on one column, and which of the two answers the question you were asked.

statistics practice Statistics

Apr 3, 2025

The top of pm25 is 2.7× the 99th percentile

A max 2.7× the 99th percentile, and the three different things that could be.

statistics quality Statistics

Apr 2, 2025

What cancelled actually contains in flight-delays

8,000 rows, 2 distinct values, and the one fact about cancelled that changes how you query it.

quality practice Data quality

Apr 1, 2025

r = 0.92 between distance_km and duration_min

A measured correlation of 0.92 between two columns, what its square says, and the diagnostic that costs one line.

statistics Statistics

Mar 31, 2025

Charting temp_c by site

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

visualization Visualization

Mar 30, 2025

Reading subject_id before trusting it in clinical-trial

900 rows, all distinct, and the one fact about subject_id that changes how you query it.

quality practice Data quality

Mar 29, 2025

An exercise on city-air-quality

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

Mar 28, 2025

Reader question — 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

Mar 27, 2025