# Sampling Techniques ~ Random, Systematic and Stratified

Refers to AQA A2 Geography

Unit 4A - Hasn't come up since June 2010 so might be one that's going to come up soon.

- Created by: Bethany
- Created on: 15-06-14 18:00

First 397 words of the document:

SAMPLING TECHNIQUES

WHAT IS SAMPLING?

1. Sampling is a shortcut method for investigating a whole population.

2. Data is gathered from a small part of the whole parent population or sampling frame, and used to

inform what the whole picture is like.

WHY DO WE NEED TO SAMPLE? WHY NOT USE DATA FROM THE WHOLE POPULATION?

1. In reality, especially for a business that needs to make a profit, there is simply not enough time,

energy, equipment, labour and money to measure every single item/ site within the parent

population or whole sampling frame.

2. This means that we have to use an appropriate sampling technique to obtain a representative,

statistically valid, sample of the whole.

WHAT CONSIDERATIONS DO WE NEED TO MAKE BEFORE WE TAKE A SAMPLE?

1. The larger the sample size, the more accurate the representation of the whole population/ site.

2. The sample size chosen needs to be a balance between obtaining a statistically valid representation,

and the time, energy, money, labour, equipment and access available.

3. A sample with the least bias is the most statistically valid.

4. Most sampling approaches assume the parent population has a normal distribution where many

items are clustered close to the mean, with few extremes.

5. A 95% probability or confidence level is usually assumed, for example 95% of the population lie

within two standard deviations of the mean.

6. This also means that up to 5% lie outside this. Therefore, sampling however good it claims to be, can

only be a very close estimate.

SAMPLING TECHNIQUES

There are three main types of sampling strategy...

Random

Systematic

Stratified

You may then need to decide on a point, line or area method.

RANDOM SAMPLING

This is the least bias of all the sampling techniques, are there is no subjectivity. Each member of the

total population has an equal chance of being selected.

They sample can be obtained using a random number table Microsoft Excel has a function enabling

this to be carried out.

RANDOM POINT SAMPLING

1. A grid is drawn over a map of the study area.

2. Random number tables are used to obtain coordinates or grid references for the points.

3. Sampling takes places as feasibly close to these points as possible.

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