Leo+DadMade for Leo
What Is Data
Rung 3 of 3 · The traps

Where It Gets Tricky

The two questions work — until a number turns out to be a costume. Here are the disguises that fool everyone.


Spot the trap Six claims, one at a time. Decide if each is true to the model or a trap, then check the reasoning.
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The two questions are solid, but they ask “is it a number?” — and some labels are written as numbers just to fool you. Here's how to see through the costume.

Numbers That Are Really Categories

A postcode is 2000. A jersey number is 7. A phone number is a long string of digits. They're written as numbers, so “is it a number?” tries to say numerical — but think about what the number does. Postcode 2000 isn't twice as much as postcode 1000; it's just a name for a place that happens to be spelled in digits. The real test isn't “does it look like a number?” — it's “can you sensibly do maths on it?” The average of two jersey numbers is meaningless. So postcodes, jersey numbers and phone numbers are all categorical, every time.

Say it plainly: don't ask “does it look like a number?” — ask “would averaging it mean anything?” If averaging it is nonsense, it's a label, not a number.

Ordered Labels Hiding in Plain Sight

Some categories aren't just buckets — they come in an order. Small / medium / large. Cold / warm / hot. Disagree / neutral / agree. These are still categorical (you can't average “medium”), but unlike eye colour they have a built-in ranking. They get their own name — ordinal — for exactly that reason: the order carries real meaning, even though there's no number underneath. Plain unordered labels like eye colour are nominal; ordered ones are ordinal.

Age, the Famous Trick

We say “I'm 13,” a whole number, so age looks discrete. But age is continuous — you are constantly, smoothly getting older; right now you're 13 years, some months, some days, some seconds. We just round it down to whole years because that's convenient. Rounding a value for convenience never changes its real type. Height rounded to the nearest centimetre is still continuous; age rounded to whole years is still continuous. The type lives in how the quantity behaves, not in how neatly we wrote it down.

A Worked One, Slowly

Is a student's ID number numerical or categorical? It's written as a number, so the lazy answer is “numerical.” But run the real test: would averaging two student IDs mean anything? No — ID 4071 isn't “more” than ID 1207, it's just a name in digits. So it's categorical, same family as postcodes and jerseys. Costume off.

Us, Thinking Out Loud

Your house number is “numerical-looking” — would adding two house numbers together ever mean anything?

“T-shirt size” and “eye colour” are both categorical — what does one of them have that the other doesn't?