Collecting data and sampling · Grade 7
In Grade 7 you learn: populations and samples; random sampling.
To learn about a whole population, you usually collect data from only a sample. A good sample is big enough and represents the population fairly. A biased sample gives misleading results, however carefully you analyse it.
Key facts
- Population: everyone or everything you want to find out about. Sample: the part you actually collect data from.
- Simple random sample: every member has an equal chance of being chosen.
- Systematic sample: pick a random start, then take every kth member of a list. k = population ÷ sample size.
- Bigger random samples give more reliable results.
Worked examples
Example 1
A gym has 600 members. Describe how to take a systematic sample of 30.
Answer: Pick a random start from 1 to 20, then every 20th member
Example 2
40 fish are caught, tagged and released. Later 50 fish are caught and 8 of them have tags. Estimate the number of fish in the lake.
Answer: About 250 fish