Sampling and Normal Distribution
What do you need to be able to do?
- Comparing populations and samples
- The capture-recapture technique
- The normal distribution
- Population: total number of items being studied
- Census: survey of ALL the population
- Sample: a smaller group of the population
Main types of samples:
- Random sample: each item equally likely to be chosen from the population,
- Systematic sample: items chosen on regular intervals, e.g. every tenth car
- Stratified sample: a random sample is taken from each stratum/layer of a population. E.g. if a population contains 70% adults and 30% children then sample will contain 70% adults and 30% children
3. Capture-Recapture Technique
Capture-recapture technique is a way of estimating the size of a population.
Estimated Population = (number tagged x number captured) / (number tagged and recaptured)
What a video here
4. Normal Distribution
Normal distribution is a bell-shaped curve.
- 68% of scores lie within one standard deviation from the mean
- 95% of scores lie within two standard deviations of the mean
- 99.7% of scores lie within three standard deviations of the mean
A z-score (standardised score) shows the position of a 'raw' score relative to the mean.
The distribution of z-scores is a normal distribution with a mean of 0 and a standard deviation of 1