Topics · Primary Science · Diversity: Classification
Classification: sorting by what is true
Classifying means putting things into groups by a property they either have or do not have. The grouping is only useful if the question dividing them has a yes-or-no answer, and that is what most marks turn on.
- 414 exam papers analysed
- appears in 93% of P3 Science papers
- ~3 min read
01 A good sorting question
A classification question must split everything into two groups with nothing left over and nothing in both.
“Does it have a backbone?” works — every animal either does or does not.
“Is it big?” does not. Big compared with what? Two people would sort the same animals differently, so the grouping carries no information.
So the test for a sorting question is: could two people disagree about the answer? If yes, it is not good enough.
02 Using a key
Start at the top and follow the yes or no answers down until you reach a name. Every step removes half the possibilities.
- Start at the first question, never in the middle.
- Answer for the one thing you are identifying, not for the group it might belong to.
- Follow the branch you answered, and read the next question on that branch only.
- Stop when you reach a name rather than another question.
- Check backwards — the thing you identified should satisfy every answer you gave along the way.
03 The groups that get tested
Papers use a small set of groups, and the boundaries between them are where the marks are.
Vertebrates have a backbone: mammals, birds, reptiles, amphibians, fish. Invertebrates do not: insects, spiders, worms, snails.
Insects have six legs and three body parts. A spider has eight legs and is not an insect — the commonest single error on this topic.
Mammals have hair or fur and feed their young on milk. A whale and a bat are both mammals, which papers use deliberately because one swims and one flies.
Amphibians live in water when young and on land as adults; reptiles have dry scaly skin and lay eggs on land.
| Animal | Backbone | Feathers | Six legs | Fur or hair |
|---|---|---|---|---|
| Sparrow | ✓ | ✓ | ✗ | ✗ |
| Bat | ✓ | ✗ | ✗ | ✓ |
| Ant | ✗ | ✗ | ✓ | ✗ |
| Spider | ✗ | ✗ | ✗ | ✗ |
04 Building your own key
The harder question gives you four or five things and asks you to write the key. The method is the same each time.
- Find a property that splits the set roughly in half, and that everything clearly has or lacks.
- Write it as a yes-or-no question. Not “what colour is it”, which has many answers.
- Repeat inside each group until every group holds one thing.
- Check every item runs through cleanly — if something could go either way at any question, that question is not good enough.
05 How to say them
These are words most children have never said out loud, and a word you cannot say is a word you avoid using. Capitals mark the syllable you lean on. Tap the speaker to hear it.
- classifysayKLASS-i-fy
To put things into groups by a property they share.
- vertebratesayVER-tuh-bruhtmore than onevertebrates
An animal with a backbone: mammals, birds, reptiles, amphibians and fish.
- invertebratesayin-VER-tuh-bruhtmore than oneinvertebrates
An animal without a backbone: insects, spiders, worms, snails.
- classification keymore than oneclassification keys
A set of yes-or-no questions that leads to the name of a thing.
- mammalsayMAM-uhlmore than onemammals
An animal with hair or fur that feeds its young on milk. Whales and bats are mammals.
- amphibiansayam-FIB-ee-uhnmore than oneamphibians
An animal that lives in water when young and on land as an adult, like a frog.
06 What each level is tested on
The same topic, asked differently as it goes up. The percentage is the share of papers at that level in which it appears.
- P3
Sorting living things into groups, and the main animal and plant groups.
Practise P3 Diversity: Classification — 10 questions →93% of P3 papers
- P4
Using a classification key, and the properties that define each group.
Practise P4 Diversity: Classification — 10 questions →70% of P4 papers
- P5
Building a key, and classifying by more than one property at a time.
Practise P5 Diversity: Classification — 9 questions →53% of P5 papers
- P6
Classification inside longer questions, and judging whether a sorting question is good enough.
Practise P6 Diversity: Classification — 9 questions →68% of P6 papers
07 Mistakes worth knowing about
Each of these is wrong. Decide why before you open it.
“A spider is an insect.”
Insects have six legs; a spider has eight. Both are invertebrates, but only one is an insect.
“A whale is a fish because it lives in the sea.”
It has a backbone, hair and feeds its young on milk, so it is a mammal. Where an animal lives is not what sorts it.
“Is it big?” is a fine sorting question.
Two people would answer it differently. A sorting question must have an answer everyone agrees on.
“A bat is a bird because it flies.”
It has fur and feeds its young on milk, so it is a mammal. Feathers are what make a bird.
“I started halfway down the key because I knew it was an insect.”
Start at the top every time. Skipping ahead assumes the answer you were asked to find.
Now try it
38 questions across 4 levels, marked as you go with the working shown.