Sampling and Bias
Good samples represent the population.
About this topic
Statisticians study a sample to learn about a population. A good sample is random and representative, using methods such as simple random, stratified, cluster or systematic sampling. Bias arises from flawed methods, such as convenience samples, voluntary response or leading questions.
In a simple random sample every member has an equal chance of selection. A stratified sample splits the population into groups, such as grades in a school, and samples from each in proportion. Larger samples reduce random error but never fix bias: a huge but biased sample is still wrong. Margin of error describes how far a sample result is likely to be from the truth, shrinking as the sample size grows.
Famous example: in 1936 a magazine poll of millions of readers predicted the wrong winner of a US presidential election because its sample, drawn from phone and car owners, favoured wealthier voters. A survey asking about school lunches only at the cafeteria misses students who bring lunch.
Sampling is used in polls, medical trials and market research. Tips: ask who was left out, how the question was worded and who chose to respond, and remember that a poll result applies only to the population it sampled.
Key ideas
- Random samples reduce bias
- Larger samples reduce error
- Surveys can be biased by wording
Quick check
Use cases: where Sampling and Bias helps
- Homework: review the key idea behind Sampling and Bias before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Sampling and Bias without notes.
- Visual learning: play with the interactive lab to see how Sampling and Bias changes when you adjust its inputs.
- Teaching a friend: explain Sampling and Bias in your own words, then compare your version with the key ideas.
Number and algebra skills are used in science, engineering, budgeting and computing.
Frequently asked questions
What is Sampling and Bias?
Good samples represent the population.
What will I learn in Sampling and Bias?
Random samples reduce bias. Larger samples reduce error. Surveys can be biased by wording.
Is Sampling and Bias part of Mathematics?
Yes. It is one of 200 Mathematics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Sampling and Bias lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Sampling and Bias?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.