Numbers That Don't Lie: Percentages, Averages and Rates at Work
By the end, you'll ask any number five questions (Out of what total? From what to what? How many cases are behind it? Which typical number did they use? How precise can it be?) and know which figures you can sign off on.
You go at your own pace, with your Maestro beside you, 1:1, through the whole course: ask it anything, and it asks you questions too.
13 skills · 4 modules
What will you be able to do?
Each box is a skill. The base is at the bottom, recognizing and explaining, and you work your way up to building something of your own and judging what works and why. Hover over a box to see what it builds on. Click it to see where I teach it and how your Maestro checks it.
- Evaluate
- Analyze
- Apply
- Understand
Evaluate
Choose between the mean and the median for the question at hand, and justify it
Where I teach it: The typical number · When should you use the mean, and when the median?
Builds on:
- Describe how values are spread: where they cluster and which ones sit far away
- Explain why the mean and the median pull apart
Your Maestro asks you to judge when it fits and why and saves it as evidence when you do.
Check the number in a headline or report before believing it
Where I teach it: Before you believe a number · What should you ask a number before you believe it?
Builds on:
- Tell a change in percentage points from a change in percent
- Compare a relative change with the same change in units
- Recognize when a few cases make a number swing a lot
- Choose between the mean and the median for the question at hand, and justify it
- Combine the rates of several groups by adding first and dividing after
- Count how many alerts are real when what you're looking for is rare
- Round a number to the precision its data allows
Your Maestro asks you to judge when it fits and why and saves it as evidence when you do.
Analyze
Compare a relative change with the same change in units
Where I teach it: Comparing places and changes · Did it double, or go from 3 to 6? Relative vs. absolute change
Builds on:
- Calculate a percent change by dividing by the starting value
Your Maestro asks you to break a problem down with it and saves it as evidence when you do.
Recognize when a few cases make a number swing a lot
Where I teach it: Comparing places and changes · Why do small towns have the highest (and lowest) rates?
Builds on:
- Calculate a rate per 100,000 people to compare places of different sizes
- Compare a relative change with the same change in units
Your Maestro asks you to break a problem down with it and saves it as evidence when you do.
Describe how values are spread: where they cluster and which ones sit far away
Where I teach it: The typical number · When should you use the mean, and when the median?
Builds on:
- Explain why the mean and the median pull apart
Your Maestro asks you to break a problem down with it and saves it as evidence when you do.
Apply
Calculate a percent change by dividing by the starting value
Where I teach it: Percentages: out of what total? · If something goes up 50% and then down 50%, are you back where you started?
Your Maestro asks you to use it on a new case and saves it as evidence when you do.
Tell a change in percentage points from a change in percent
Where I teach it: Percentages: out of what total? · Did it go up 2% or 2 points? Percentage points vs. percent
Builds on:
- Calculate a percent change by dividing by the starting value
Your Maestro asks you to use it on a new case and saves it as evidence when you do.
Calculate a rate per 100,000 people to compare places of different sizes
Where I teach it: Comparing places and changes · How do you calculate a rate per 100,000 people?
Builds on:
- Identify what total a percentage is out of
Your Maestro asks you to use it on a new case and saves it as evidence when you do.
Combine the rates of several groups by adding first and dividing after
Where I teach it: The typical number · Can you average percentages?
Builds on:
- Identify what total a percentage is out of
Your Maestro asks you to use it on a new case and saves it as evidence when you do.
Count how many alerts are real when what you're looking for is rare
Where I teach it: Before you believe a number · If the system catches 90% of frauds, what does an alert mean?
Builds on:
- Identify what total a percentage is out of
Your Maestro asks you to use it on a new case and saves it as evidence when you do.
Round a number to the precision its data allows
Where I teach it: Before you believe a number · How many decimal places should a percentage have?
Your Maestro asks you to use it on a new case and saves it as evidence when you do.
Understand
Identify what total a percentage is out of
Where I teach it: Percentages: out of what total? · 43% of what? How to tell what total a percentage is out of
Your Maestro asks you to explain it in your own words and saves it as evidence when you do.
Explain why the mean and the median pull apart
Where I teach it: The typical number · Is the average salary what most people earn?
Your Maestro asks you to explain it in your own words and saves it as evidence when you do.
Who is this course for?
For anyone who receives, calculates, or publishes numbers at work: sales reports, budgets, news stories, presentations. If you ever hesitated between saying "it rose 2%" or "it rose 2 points", it's for you. Adding, subtracting, multiplying, and dividing is enough. You don't need statistics or coding.
Ask your Maestro
Your Maestro is an AI that knows this course. Ask it about the first page and it answers right here, free.
You: Where do I start?
Your Maestro: From what you already know. Tell me what you've done with data, and I'll start this page from there.
Asking about: Answers from the page "If something goes up 50% and then down 50%, are you back where you started?"
Only this browser keeps what you ask here. How it works
What we'll cover
Percentages: out of what total?
Comparing places and changes
The typical number
Before you believe a number
How long does it take?
45 minutes to 1.3 hours
A guide: you go at your own pace, and the course waits where you left off.
Common questions
- What is the difference between percentage points and percent?
- If something goes from 10% to 12%, it rose 2 percentage points, but it rose 20% relative to where it started. Points measure the distance between two percentages. Percent measures the change against the starting point. Saying "it rose 2%" without clarifying which one is among the most common mix-ups.
- When do I use the median instead of the average?
- Use the median when a few very high values pull the average up, like one huge salary or one giant branch. The median is the middle value and barely moves because of extremes. The course shows you how to swap an average for a median and which typical number better represents your data.
- Do I need to know statistics or math?
- No. Adding, subtracting, multiplying and dividing is enough. The course uses no hypothesis tests, p-values or regressions: it teaches you to read percentages, averages and rates correctly, because those are the numbers everyday reports and news stories get wrong most often.
- Do I need to know how to code?
- No. The exercises come with almost all the code written, and you change one or two lines: a division, a mean swapped for a median. Everything runs right on the page, with nothing to install. In the final challenge you try writing a query, but you can ask your Maestro for a template and explain your critique in words before SQL.
- Do I need a course, or can I learn this on my own?
- You can learn it on your own, and many people do. A course gives you the order: what to practice first, ready-to-use data, and exercises that show where you go wrong before it happens at work. This course is free, and your Maestro answers questions as you go. If you prefer going solo, the method still works.
- Is it free? Do I need an account?
- It's free. You can read every page without an account. With a free account you enroll in one tap, run the exercises and ask your Maestro.
- Is the data real?
- No. Tiendas La Ceiba, its branches, its salaries and the municipalities where it operates are all made up for the course (synthetic data). They're built so the traps you see in real data show up: one giant store, one very high salary, a town of 2,450 people.
- Who is this course for?
- For anyone who receives, calculates or publishes numbers at work: sales reports, budgets, news stories, presentations. If you ever hesitated between saying "it rose 2%" or "it rose 2 points", it is for you. Adding, subtracting, multiplying and dividing is enough. You need no statistics and no coding.
How do I check what AI does? I teach it in Your AI and Your Data: Don't Let It Make Up the Number and Check what AI gives you in three steps.
What you get
- 4 modules and 12 pages, open whenever you want them.
Open to everyone: read all of it, and your Maestro, an AI, answers your questions on every page.
With a free account: your Maestro for the whole course, always on the page you're on, and the exercises run with nothing to install.
Words you'll meet
In the lessons, tap one to see what it means, or ask your Maestro about it.
Who's teaching you
Sergio Sánchez
I'm Sergio, chekos online. I'm a data architect, and I've spent more than ten years working with data in the Bay Area, mostly at mission-driven organizations: policy research first, then education, and now early childhood services. I grew up in Tijuana. In 2018 I started tacosdedatos to share what I was learning, and since then it's been a blog, a podcast, courses, guides, and a community. These days I spend a lot of my time building with AI agents and writing about how I organize my work around them. I share what I've learned; you take what's useful and make it yours.
See you in the course
Numbers That Don't Lie: Percentages, Averages and Rates at Work