Types of sampling

Question 1

Population Definition

Answer 1

The whole group, consisting of every single person/animal/animal etc. that you want to investigate.

Question 2

What is a finite population?

Answer 2

A population is finite if it’s possible for someone to count how many members there are.

Question 3

What is an infinite population?

Answer 3

Populations are said to be infinite if it’s impossible to know exactly how many members there are - there won’t literally be infinitely many members, just too many to count

Question 4

Census definition

Answer 4

When you collect information from every member of a population. It’s a survey of the population of the whole population.

Question 5

Advantage of a Census

Answer 5

It’s an accurate representation of the population because every member has been surveyed - it’s unbiased.

Question 6

Disadvantages of a Census

Answer 6

1. For large populations, it takes a lot of time and effort to carry out. 2. This can make it expensive to do. 3. It can be difficult to make sure all embers are surveyed. If some are missed it will be biased. 4. If the tested items are used up or damaged in some way it’s going to be impractical.

Question 7

Sample definition

Answer 7

When doing a Census is impractical, you can just question a selection of the people or items. This group is called a sample.

Question 8

Sampling frame definition

Answer 8

A full list of all the sampling units. It must give a unique name or number to each sampling unit.

Question 9

Advantages of a sample

Answer 9

1. Sample surveys are quicker and cheaper than a census, and it’s easier to get hold of all the required information. 2. It’s the only option when surveyed items are used up or damaged.

Question 10

Disadvantages of a sample

Answer 10

1. There’ll be variability between samples - each possible sample will give different results, so you could just happen to select one which doesn’t accurately reflect the population 2. Samples can easily be affected by sampling bias.

Question 11

How to avoid bias?

Answer 11

1. Select from the correct population and make sure that no one is excluded. 2. Select your sample at random. 3. Make sure all members respond.

Question 12

Simple Random Sampling method

Answer 12

1. Give a number to each population member from a full list. 2. Generate a list of random numbers and match them to the numbered members to select your sample.

Question 13

Advantage of Simple Random Sampling

Answer 13

Every member of the population has an equal chance of being selected, so it’s completely unbiased.

Question 14

A disadvantage of Simple Random Sampling

Answer 14

It can be inconvenient if the population is spread over a large area - it might be difficult to track down certain members.

Question 15

Systematic Sampling definition

Answer 15

Choosing every nth member of a population.

Question 16

Systematic sampling method

Answer 16

1. Give a number to each population member, from a full list of the population. 2. Calculate a regular interval to use by dividing the population size by the sample size. 3. Generate a random starting point that is less than or equal to the size of the interval. The corresponding member of the population is the first member of your sample 4. Keep adding the interval to the starting point to select your sample.

Question 17

Advantages of systematic sampling

Answer 17

1. It can be used for quality control on a production line - a machine can be set up to sample every nth item. 2. It should give an unbiased sample

Question 18

A disadvantage of systematic sampling

Answer 18

The regular interval could coincide with a pattern e.g every nth item is faulty if testing every nth item they will either all be faulty or none of them will be faulty. The sample will be biased.

Question 19

Stratified Sampling Definition

Answer 19

If the population is divided categories, you can use a stratified sample - this uses the same proportion of each category in the sample as there is in the population

Question 20

Stratified Sampling Method

Answer 20

1. Divide the population into categories. 2. Calculate the total population. 3. Calculate the number needed for each category in the sample using formula. 4. select the sample of each category at random.

Question 21

Formula to determine the size of a category in a stratified sample.

Answer 21

(size of category in population/total population)*total sample size

Question 22

Advantage of Stratified sampling

Answer 22

1. If the population has disjoint categories this is likely to give you a representative sample. 2. It’s useful when results may vary depending on categories.

Question 23

Disadvantages of Stratified sampling

Answer 23

It can be expensive because of the extra detail involved.

Question 24

Quota sampling method

Answer 24

1. Divide the population into categories. 2. Give each category a quota (number of members to sample) 3. Collect data until the quotas are met in all categories (without using random sampling)

Question 25

Advantages of quota sampling

Answer 25

1. It can be done when there isn’t a full list fo the population. 2. The interviewer continues until all the quotas are met, non-response is less of a problem.

Question 26

Disadvantages of quota sampling

Answer 26

It can be easily biased by the interviewer -within quotas, the interviewer may exclude some of the population.

Question 27

Opportunity sampling

Answer 27

This is where the sample is chosen from a selection of the population that is convenient for the sampler.

Question 28

Advantages of opportunity sampling

Answer 28

Data can be gathered quickly and easily.

Question 29

Disadvantages of opportunity sampling

Answer 29

It isn’t random and can be very biased - there is no attempt to make the sample representative of the population.