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Year 8AC9M8ST01

How Data Is Collected

Investigate techniques for data collection including census, sampling, experiment and observation, and explain the practicalities and implications of obtaining data through these techniques. Analyse and report on the distribution of data from primary and secondary sources using random and non-random sampling techniques to select and study samples.

Where a number came from matters more than the number. Four ways to collect data, each with a cost and a weakness.

Method

Census asks everyone
Complete and accurate, but expensive and slow.
Sample asks some
Fast and cheap, but only as good as the sample.
Experiment changes something
You control a variable and measure the effect.
Observation watches without asking
No one can misreport, but you cannot ask why.
Primary is yours, secondary is someone else's
Secondary data is quicker but you did not control how it was gathered.

Worked example

e.g. You want to know the average height of Year 8 students in Australia. Which technique?\text{You want to know the average height of Year 8 students in Australia. Which technique?}
  1. Consider a census. Measure every Year 8 student in the country.accurate but impossible\text{accurate but impossible}
  2. Consider observation. You cannot measure height by watching.not workable here\text{not workable here}
  3. Consider an experiment. There is nothing to change.not applicable\text{not applicable}
  4. Choose a sample, and say how it will be chosen.random sample across many schools\text{random sample across many schools}
  5. State the trade-off honestly.faster and cheaper, but carries uncertainty\text{faster and cheaper, but carries uncertainty}

Practice

Name the technique, then say what it costs you.

1
A national population count. Which technique?\text{A national population count. Which technique?}
AnswerCensus.\text{Census.}
2
Testing whether a fertiliser helps plants grow. Which?\text{Testing whether a fertiliser helps plants grow. Which?}
AnswerExperiment.\text{Experiment.}
3
Counting cars at an intersection. Which?\text{Counting cars at an intersection. Which?}
AnswerObservation.\text{Observation.}
4
You use figures from a government website. Primary or secondary?\text{You use figures from a government website. Primary or secondary?}
AnswerSecondary.\text{Secondary.}
5
Why is a census rarely used?\text{Why is a census rarely used?}
AnswerIt is slow and expensive.\text{It is slow and expensive.}
6
One weakness of secondary data?\text{One weakness of secondary data?}
AnswerYou did not control how it was collected.\text{You did not control how it was collected.}
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