Samples And Central Limit Theorum Flashcards
What is a population
Large group to be sampled
Everyone who fits the criteria, which must be clearly defined
What is a sample
Small group selected from the population
Representative sample allows generalisation
Must mini sampling error and variability
What are the three types of error in sampling
Sampling variability
Sampling error
Non sampling error
What is sampling variability
If you take repeated samples from the same population, means and sd will be different.
What is sampling error
An estimate from a sample, such as a mean will not be the same as the population value being estimated
What is a non sampling error
Poor questions, coding ect
What are the two main sampling strategies
Simple random
Stratified
What is a simple random sample
Every person in population has same chance of being selected
What is a stratified sample
Aims to represent all strata within the population.
Age sex ect
What does the sample size depend on
Variability among members of the population:
Higher variation (sd) need larger sample size
Cost
Level of precision needed for estimating
What is a sample means
Mean of a sample, may have 5 sample groups, producing 5 means
You can then produce a mean of the sample means
How do sample means and means of sample means relate to the population
The distribution of sample means will approximately follow the normal distribution
The mean of sample mean will approximately equal the true population mean
What is standard error and how is it calculated
Standard deviation of the standard means
SE = SD of population /square root of sample size
What is central limit theorem
If samples of size n are selected from a population, the means of the samples are approximately distributed with
Mean = population mean
SE = sd of population/ square root of n
How large of a sample is needed for central limit theorem to work
If distribution of population is normal, 10 or more
If skewed, 25 or more
What is the use of central limit theorem
See whether a sample mean is different from the population due to chance (sampling error) or whether the sample is actually quite different from the population
Allows you to test the the precision of a sample and forms the basis of hypothesis testing
How do you calculate the z score of standard error
Z = sample mean - population mean / standard error
What are the steps of using the sampling distribution for the mean
- Find mean and standard error of sample means
- Draw a diagram
- Calculate z score
- Look up probability on norms table and draw a conclusion
What are the two broad categories of sampling
Probability sampling methods (random)
Non probability (not random)
What are the different probability sampling methods
Simple random
Stratified
Systematic
Multistage
What are the different non probability sampling methods
Judgement or purposive or deliberate sampling
Convenience sampling
Snowball
Quota
What are the advantages of probability sampling
- requires detailed info about the population to be effective
- precisely measured, unbiased estimates
- can evaluate the relative effectiveness of various sample designs when probability sampling is used
What are the disadvantages of probability sampling
Skill needed
Time to plan and determine sample
Costs higher
What is systematic sampling
Every nth number
- simple, less variability
