# Sampling and Normal Distribution

### Chapter 11

## What do you need to be able to do?

- Sampling
- Comparing populations and samples
- The capture-recapture technique
- The normal distribution
- z-scores

## 1. Sampling

Key terms

**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

## 2. Comparing samples and population

You need to know:

## 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

## 5. Z-Scores

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