Upload
cahird
View
379
Download
4
Tags:
Embed Size (px)
Citation preview
Fieldwork skills
Learning Objectives
• ALL – to know different sampling methods
• MOST – reflect on advantages and disadvantages of sampling
• SOME – consider the effects of bias
Sampling
KWLR TABLEWhat I KNOW
about samplingWhat I WOULD
LIKE to know about sampling
What I have LEARNT about
sampling
The RANK order of what I have learnt today is
(the most important)
Complete the first two columns of your KWL
grid.
Learning Objectives
• ALL – to know different sampling methods
• MOST – reflect on advantages and disadvantages of sampling
• SOME – consider the effects of bias
Why do we sample?1. it is quicker than
measuring every item
2. it is therefore much cheaper
3. often it is impossible to measure everything
4. it is unnecessary to measure everything
5. it is sometimes impossible to gain access to everythingLearning Objectives
• ALL – to know different sampling methods
• MOST – reflect on advantages and disadvantages of sampling
• SOME – consider the effects of bias
The size of the sample will usually be dictated by the time and resources available. BUT the larger the sample the more likely it is to find a true picture of the data you are sampling.
Learning Objectives
• ALL – to know different sampling methods
• MOST – reflect on advantages and disadvantages of sampling
• SOME – consider the effects of bias
Smarties samplingYou will be conducting a mini investigation to
practise sampling. You will be using smarties to demonstrate the different sampling techniques. Do not eat your smarties as you can’t carry out the investigation without them – make it to the end and then you can eat them!!
Stretch and challenge – for each type of sampling method you must attempt to write up your own definition of this technique.
Random samplingRandom sampling means that members of a 'population' have equal chances of being selected.To carry out this type of sampling, you will need to use a table of random numbers. Random numbers can also be generated using a calculator or computer.
Learning Objectives
• ALL – to know different sampling methods
• MOST – reflect on advantages and disadvantages of sampling
• SOME – consider the effects of bias
Lay out your smarties in a row. Assign each smartie a number from 1- 20(we will deal with a population of
20 smarties).
A random number calculator will generate
numbers to identify which smarties will be part of the
random sample.
Random sampling
Learning Objectives
• ALL – to know different sampling methods
• MOST – reflect on advantages and disadvantages of sampling
• SOME – consider the effects of bias
Advantages DisadvantagesNo human bias involved in the selection process.
If the sample size is small we may by bad luck obtain an unrepresentative result
KWLR TABLEWhat I KNOW
about samplingWhat I WOULD
LIKE to know about sampling
What I have LEARNT about
sampling
The RANK order of what I have learnt today is
(the most important)
Learning Objectives
• ALL – to know different sampling methods
• MOST – reflect on advantages and disadvantages of sampling
• SOME – consider the effects of bias
Add 3 things you have learnt
to your table
Systematic samplingSampling data at equal distances apart e.g. every fifty meters or every 10th person.
Learning Objectives
• ALL – to know different sampling methods
• MOST – reflect on advantages and disadvantages of sampling
• SOME – consider the effects of bias
Lay out your smarties in a row. Assign each
smartie a number from 1- 20(we will
deal with a population of 20 smarties).
Take every 5th smartie – this will be your
systematic sample.
Systemaic sampling
Learning Objectives
• ALL – to know different sampling methods
• MOST – reflect on advantages and disadvantages of sampling
• SOME – consider the effects of bias
Advantages DisadvantagesEnsures complete coverage of the sample area.
If the sample size is small we may by bad luck obtain an unrepresentative result
Stratified sampling'Strata' means 'layer'. A stratified sample is made up of different 'layers' of the population, for example, selecting samples from different age groups.
Learning Objectives
• ALL – to know different sampling methods
• MOST – reflect on advantages and disadvantages of sampling
• SOME – consider the effects of bias
All the sweets are smarties but each smartie has a different colour and the colour represents a subjection of the
whole population. Select out all the red smarties these are the stratified sample.
Stratified sampling
Learning Objectives
• ALL – to know different sampling methods
• MOST – reflect on advantages and disadvantages of sampling
• SOME – consider the effects of bias
Advantages DisadvantagesHelps to reduce bias because all sub groups of the population are included in the sample
Sometimes difficult to identify a sub group or strata
KWLR TABLEWhat I KNOW
about samplingWhat I WOULD
LIKE to know about sampling
What I have LEARNT about
sampling
The RANK order of what I have learnt today is
(the most important)
Learning Objectives
• ALL – to know different sampling methods
• MOST – reflect on advantages and disadvantages of sampling
• SOME – consider the effects of bias
Add 3 things you have learnt
to your table
Test time
Learning Objectives
• ALL – to know different sampling methods
• MOST – reflect on advantages and disadvantages of sampling
• SOME – consider the effects of bias
Q. Collecting sediment samples every 10m on a beachA. SystematicQ. In a beach questionnaire ensuring that participants both young and old are asked.A. StratifiedQ. Numbers are generated using a computer and these numbers are used as grid reference co ordinates for the site of studyA. RandomQ. In a sand dune investigation choosing a sample where there is a break of slope rather than at equally spaced intervalsA. Stratified
Avoiding bias
Learning Objectives
• ALL – to know different sampling methods
• MOST – reflect on advantages and disadvantages of sampling
• SOME – consider the effects of bias
Select out your favourite colour smarties and eat them – this
would be a bias sample! Why?
Stretch and
Challenge
Where can I find out
more about this
topic?
KWLR TABLEWhat I KNOW
about samplingWhat I WOULD
LIKE to know about sampling
What I have LEARNT about
sampling
The RANK order of what I have learnt today is
(the most important)
Learning Objectives
• ALL – to know different sampling methods
• MOST – reflect on advantages and disadvantages of sampling
• SOME – consider the effects of bias
Add 2-4 things you have learnt this lesson and
decide which is the most important thing you have
learnt or rank order everything you have
learnt from top to bottom.