A statistic is a claim about a whole group made from a handful of it, and it is only worth anything if that handful could have been anybody. These free printable worksheets give fifteen ways of choosing a sample and ask which ones are fair.
The question is not who WAS asked — it is who could never have been. Asking students in the parking lot how they get to school can never reach the ones who walked; phoning a town on landlines cannot reach a household without one. That is coverage bias, and it is visible the moment you ask who is missing.
Why this sheet works
The harder half is voluntary response, because the sample looks open. A magazine prints a survey and everybody could reply; the people who do are the ones who felt strongly enough to find a stamp. A form emailed to all 400 staff that 40 choose to answer is not a random 40 — they picked themselves. The page names both failures separately rather than lumping them into “biased”, because a student who can only say “biased” cannot say how to fix the survey.
How this one works
One question from a sheet like this one, worked a step at a time — with a sentence saying what just happened and why.
Could this sample have been anybody? Circle your answer.
1
A computer picks 200 customer numbers at random from the full list.
Yes — it is randomNo — a group is left outNo — only volunteers reply
2
A website asks visitors to click if they like the new design.
Yes — it is randomNo — a group is left outNo — only volunteers reply
3
To find how students travel, only those in the parking lot are asked.
Yes — it is randomNo — a group is left outNo — only volunteers reply
4
Every 20th name on the complete membership list is chosen.
Yes — it is randomNo — a group is left outNo — only volunteers reply
5
Listeners are invited to phone in and say whether they agree.
Yes — it is randomNo — a group is left outNo — only volunteers reply
6
Reading habits are surveyed by asking people inside the library.
Yes — it is randomNo — a group is left outNo — only volunteers reply
Reading a study · Is the sample fair?Answer key
1. A computer picks 200 customer numbers at random from the full list. → Yes — it is random — The whole list goes in and the choosing is random: a fair sample.2. A website asks visitors to click if they like the new design. → No — only volunteers reply — Clicking is a choice, so the sample is the people who cared.3. To find how students travel, only those in the parking lot are asked. → No — a group is left out — Anyone who walked or took the bus was never going to be asked.4. Every 20th name on the complete membership list is chosen. → Yes — it is random — The list is complete and nobody's place on it is their own doing.5. Listeners are invited to phone in and say whether they agree. → No — only volunteers reply — Phoning in takes effort, so only strong opinions are heard.6. Reading habits are surveyed by asking people inside the library. → No — a group is left out — People who never visit the library cannot turn up in the sample.
