Samples And Central Limit Theorum

Question 1

What is a population

Answer 1

Large group to be sampled

Everyone who fits the criteria, which must be clearly defined

Question 2

What is a sample

Answer 2

Small group selected from the population

Representative sample allows generalisation

Must mini sampling error and variability

Question 3

What are the three types of error in sampling

Answer 3

Sampling variability
Sampling error
Non sampling error

Question 4

What is sampling variability

Answer 4

If you take repeated samples from the same population, means and sd will be different.

Question 5

What is sampling error

Answer 5

An estimate from a sample, such as a mean will not be the same as the population value being estimated

Question 6

What is a non sampling error

Answer 6

Poor questions, coding ect

Question 7

What are the two main sampling strategies

Answer 7

Simple random

Stratified

Question 8

What is a simple random sample

Answer 8

Every person in population has same chance of being selected

Question 9

What is a stratified sample

Answer 9

Aims to represent all strata within the population.

Age sex ect

Question 10

What does the sample size depend on

Answer 10

Variability among members of the population:
Higher variation (sd) need larger sample size

Cost

Level of precision needed for estimating

Question 11

What is a sample means

Answer 11

Mean of a sample, may have 5 sample groups, producing 5 means

You can then produce a mean of the sample means

Question 12

How do sample means and means of sample means relate to the population

Answer 12

The distribution of sample means will approximately follow the normal distribution

The mean of sample mean will approximately equal the true population mean

Question 13

What is standard error and how is it calculated

Answer 13

Standard deviation of the standard means

SE = SD of population /square root of sample size

Question 14

What is central limit theorem

Answer 14

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

Question 15

How large of a sample is needed for central limit theorem to work

Answer 15

If distribution of population is normal, 10 or more

If skewed, 25 or more

Question 16

What is the use of central limit theorem

Answer 16

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

Question 17

How do you calculate the z score of standard error

Answer 17

Z = sample mean - population mean / standard error

Question 18

What are the steps of using the sampling distribution for the mean

Answer 18

1. Find mean and standard error of sample means
2. Draw a diagram
3. Calculate z score
4. Look up probability on norms table and draw a conclusion

Question 19

What are the two broad categories of sampling

Answer 19

Probability sampling methods (random)

Non probability (not random)

Question 20

What are the different probability sampling methods

Answer 20

Simple random

Stratified

Systematic

Multistage

Question 21

What are the different non probability sampling methods

Answer 21

Judgement or purposive or deliberate sampling

Convenience sampling

Snowball

Quota

Question 22

What are the advantages of probability sampling

Answer 22

• 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

Question 23

What are the disadvantages of probability sampling

Answer 23

Skill needed

Time to plan and determine sample

Costs higher

Question 24

What is systematic sampling

Answer 24

Every nth number

• simple, less variability