Statistics / Unit 5: Collecting Data
5.2 Sampling Methods
Notes
A good sample is chosen by random selection so that it represents the population.
- Simple random sample (SRS): every group of $n$ individuals has the same chance of being the sample. Example: number the 500 students and draw 40 numbers at random.
- Stratified: split the population into groups (strata) of similar individuals, then take an SRS from each stratum.
- Cluster: split the population into clusters, randomly select some clusters, and survey everyone in them.
- Systematic: pick a random starting point, then every $k$th individual.
- Convenience (easiest to reach) and voluntary response (people choose to respond) are not random and are usually biased.
Practice
A principal numbers all 500 students and uses a random number generator to select 40 different numbers. This is a:
Every group of 40 students has the same chance of being chosen.
A school has four grade levels. A researcher takes a random sample of 25 students from each grade level. This is a:
The population is split into groups (grades), and an SRS is taken from each.
A researcher randomly chooses 5 of the 30 homerooms and surveys every student in those homerooms. This is a:
Entire groups (clusters) are selected at random and everyone in them is surveyed.
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