Sampling Flashcards
What does it mean if data is qualitative?
When data can be in categories eg flavour of cupcakes of place of birth
What does it mean if data is discrete?
The data can only take individual distinct values eg number of siblings or shoe size
What does it mean if data is continuous?
Something that can be measured and recorded to any accuracy eg height or weight
What is a population in sampling?
All the possible values a variable can take eg heights of all adult females
When are samples used?
If It is impossible or not advisable to investigate the whole population, usually as it would be impractical or destructive
Eg
You can’t interview everyone in the uk
And to test the lifespan of an entire population of lightbulbs would destroy all the lightbulbs
Why does the sample need to be as representative of the population as possible?
The sample will be used to draw conclusions about the population
What is a parameter?
A statistical measure of the entire population eg standard deviation or mean height of all women in uk
What is a statistic?
A statistical measure carried out on a sample eg mean height of a sample of women in the uk
Population => Parameter
Sample => Statistic
Define a truly random sample
A random sample of size n is a sample selected in such a way that all possible samples of size n have an equal chance of being selected
What is a simple random sample?
A random sample that does not allow any repetition (without replacement)
In simple random sampling how are numbers picked?
Random number generator
Give some advantages and disadvantages of simple random sampling
Advantages
Most statistical methods assume a random sample
Only method which is truly random
Disadvantages
Can be very time consuming
With large target population can be impossible to carry out
List of whole population needed
What is opportunity sampling?
Taking the sample from people who are available at the time the study is being carried out and fit the criteria you are looking for
Eg you use the first 20 people who enter the dining room
What are some advantages and disadvantages of opportunity sampling?
Advantages
May save time and money
Can be seen as adequate when investigating processes which are thought to work in similar ways for most individuals such as memory processes.
Disadvantages
Likely to produce a sample not representative of the population
Researcher may choose people from their own social and cultural group
Some people may refuse to take part making the sample self selecting
What is stratified sampling
Population is divided into relevant strata and random samples are taken from each strata in proportion to the size of the strata
From each strata a random sample will be taken
What are some advantages and disadvantages of stratified sampling
Advantages
More representative of the population than a simple random sample
Disadvantages
Prior knowledge needed to divide population into relevant strata
Could be time consuming and expensive
What is quota sampling
A stratified sample where the samples from each strata are chosen in the most convenient way
What are some advantages and disadvantages of quota sampling
Advantages
Cheap n straightforward
Can be used when population list not available
Disadvantages
Prior knowledge needed to divide population into relevant strata
Some people may refuse to take part
Bias due to investigators preference
May be hard to tell if people are part of relevant quota or not
What is systematic sampling?
Members of population are chosen at regular intervals
What are some advantages and disadvantages of systematic sampling
Advantages
May be very easy to administer
Disadvantages
List of population needed
Some members of population may not have an equal chance of being choses
What is a cluster sample
A cluster is a pre existing grouping in the population a cluster sample will study randomly chosen clusters of the population
Eg study only schools in Leicestershire
Advantages and disadvantages of cluster sampling
Advantages
Members of cluster May be geographically close saving time and money
Disadvantages
A cluster of samples may not be representative of the population. For example a post code may contain certain types of residents
When is a stratified sample better than a random sample?
If relevant prior knowledge available as sample more likely to be representative of the population whilst still including some randomness
Quota cluster and systematic samples are used to overcome the practical problems associated with random sampling but a random sampling is usually preferred to these methods
