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Section 6.2 · Statistics and probability

Taking a sample

Choose a sampling method, understand how sample size affects how much you can trust a result, and recognise bias.

6 teaching slides 75 practice questions Worked answers for every question

1 · Teaching slidesuse ← → keys

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2 · Key words

populationEvery member of the group being studied.
sampleThe part of the population you actually collect data from.
censusA survey of the whole population.
sample sizeHow many members are in the sample.
random sampleOne where every member has an equal chance of being chosen.
systematic sampleOne chosen by a fixed rule, such as every 10th name.
convenience sampleOne made of whoever is easiest to reach; it is usually biased.
biasWhen some members are more likely to be chosen than others, so the result is misleading.
representativeA sample that reflects the population fairly.

3 · Summary of the sectionrevise from this

  • The population is everyone you want to know about; the sample is who you actually ask.
  • A census asks everyone — most reliable, rarely practical.
  • A larger sample size generally gives a more reliable result, because one unusual answer matters less.
  • A larger sample does not fix bias. Bias is about who is chosen, not how many.
  • Random: everyone has an equal chance. Systematic: a fixed rule such as every 10th. Convenience: whoever is nearest — usually biased.
  • Ask where you are sampling from: a bus stop samples bus users, a football club samples football fans.
  • To estimate a whole population, find the proportion in the sample and apply it: 24 out of 60 is 0.4, so 0.4 × 900 = 360.
  • Always state your sample size when you report a result — it tells the reader how much to trust it.
PDFPrintable worksheet All 75 questions, space to work PDFWorked answers Full solutions, not just the answer

Video lessons1 videos

Math Antics - Mean, Median and Mode

American stretchmathantics

Where the data goes next. Watch it now to see why a fair sample matters before you average anything.

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4 · Practice75 questions, easiest first

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