3.2 Sampling

Question 1

Define Sampling

Answer 1

Sampling is to draw a subset of the population that truly represents the target population as a whole i.e. infers that the researcher is able to draw conclusions about the target population from the sample or subset of that target population

Question 2

Explain the importance of sampling in health research

Answer 2

Unnecessary and impractical to study the whole population
Obtain useful and valid information about the population

Question 3

What are the two main types of sampling?

Answer 3

Probability
Non-probability

Question 4

Validity

Answer 4

i.e. findings are accurate and indicative, the research measures what it set out to measure

Truth and accuracy of the data and findings.
It refers to
- concepts being investigated
- people or objects being studied
- methods by which data are collected
- findings that are produced

Question 5

True or False. The extent to which research findings can be generalised to the target population will depend on the extent of sampling error or bias within the study

Answer 5

True
i.e. ↑ error/bias →↓representativeness of findings

Question 6

Sampling frame

Answer 6

Members of a population eligible to be included in a sample

Question 7

What is a sampling plan and what is its purpose?

Answer 7

Outlines the process by which the subsets or participants are dawn from target population.

Aim is to minimise sampling error and bias and maximise representativeness of the sample.

No sampling technique can guarantee against sampling errors – we can just aim to minimise them.

Question 8

What is probability sampling?

Answer 8

Every member of the population has an equal (and independent) chance of recruitment vs. not equal chance.

Intent is to generalise the findings of the sample to the population from which it was taken.

Used when research is investigating a large population and requires a statistical description.

Helps to reduce sampling error, increase internal validity. Reduces bias as everyone has equal chance of recruitment.

Question 9

What are the 4 types of random sampling?

Answer 9

Simple random sampling
Systematic random sampling,
Stratified random sampling
Cluster random sampling

Question 10

Internal validity means….

Answer 10

Less bias
Scientific process is used
Study is accurate

Refers to the extent to which changes in the dependent variable (the observed effects) can be attributed to the independent variable, rather than EXTRANEOUS variables.

Question 11

External validity

Answer 11

Generalisation of findings from sample to population (representativeness of sample)

Refers to the degree to which results of a study are generalisable beyond the immediate study sample and setting to other samples and settings.

Question 12

True or False. You need internal validity to have external validity.

Answer 12

True

Question 13

What is simple random sampling?

Answer 13

Establish sampling frame.
Decide on sample size.
Select the required number of units starting at a random point by either lottery method or use of random numbers
e.g. drawing names/numbers out of a hat, or assigning everyone a number and using a random number generator to select

Question 14

Systematic sampling

Answer 14

Sampling frame is available.
Sample size has been predetermined.
Start at a random point on sampling frame but pick every “nth” unit (the gap)
The gap is determined by sampling frame # / sample size e.g. 25 / 5 = select every 5th persion

Question 15

Stratified random sampling

Answer 15

Subdived groups. Randomly selected from each group. Represents important characteristics of the population.

Similar to simple random sampling but the population is first subdivided into homogenous subsets (strata) from which the required number of units can be randomly selected.

Stratification reflects the population characteristics significant or crucial to the research question – e.g. gender, age, occupation.

Allows for a greater degree of representativeness of the population.

Question 16

Sampling ratio =

Answer 16

sample size to population size

Question 17

Cluster sampling

Answer 17

Utilises simple random sampling of predetermined groups or clusters of individual elements of interest; e.g. villages, hospitals, schools families, health areas

Often used when cannot create a sampling frame that will include all the units of the accessible population.

Example:
Wish to investigate attitudes to GBLTI amongst nursing home employees in WA
List all the nursing homes in WA (this becomes the sampling frame)
Randomly select a number of nursing homes
Interview the employees from each selected nursing home

Question 18

Does a larger sample size mean less or more sampling error?

Answer 18

Less

Question 19

Do samples of more diverse populations need to be smaller or larger than samples of more homogenous population?

Answer 19

Larger

Question 20

Non-probability sampling

Answer 20

The probability of a potential research participant being selected is not known in advance.
The findings cannot be generalised to a larger group of people

Question 21

Non-randomness of the sampling increases the risk of what?

Answer 21

Bias

Question 22

When is non-probability sampling commonly used?

Answer 22

Particularly when have limited resources (much less costly and time consuming than probability sampling )
or unable to identify or access all members of population.

Useful in exploratory research and qualitative studies; samples not statistically representative but can give in-depth insights into phenomena

Question 23

What are the 5 types of non-probability sampling?

Answer 23

*Purposive
*Convenience
*Snowball
Theoretical
Quota

Question 24

Purposive sampling …

Answer 24

Participants are recruited according to pre-selected criteria relevant to the research aims/questions of a given study.

Designed to recruit thosewho have the required status, experience or knowledge of interest to the researcher.

Typically choice of sample is based on judgement that it has
characteristics typical of the population.
Open to selection bias

Question 25

Convenience sampling…

Answer 25

When people are invited to participate in the study because they are conveniently (opportunistically) available with regard to access, location, time and willingness.

(Utilises the most readily available convenient units of the population (cases/persons) who fit the criteria of the study.)

Open to self selection bias
Suffer from either under-representation or over-representation
Less bias if the population is homogenous

Most common form of sampling in qualitative research.

Question 26

What are possible contributors to sampling errors in health research?

Answer 26

Accessible population is not representative of the target population; Do all groups want to participate or ‘be found’?

Sampling frame may not be representative of population → unrepresentative sample

Small sample size may over or under represent certain characteristics of the population

The researcher may not have accurately identified the defining characteristics of the population

Question 27

What does rigour mean / refer to in research?

Answer 27

Rigour refers to the trustworthiness/credibility/strength of the research

Question 28

What are traditional criteria for judging quantitative research?

Answer 28

Internal validity
External validity
Reliability
Objectivity

Question 29

What are alternative criteria for judging quantitative research?

Answer 29

Credibility (instead of int validity in quant)
Transferability (instead of ext validity in quant)
Dependability (instead of reliability in quant)
Confirmability (instead of objectivity in quant)

Question 30

What is snowball sampling?

Answer 30

When the researcher starts gathering information from one or a small number of people and then requests they put the researcher in touch with others who may be friends, relatives, colleagues or other significant contacts.

Especially useful in recruiting ‘hidden’ populations e.g. marginalised or stigmatised individuals - drug users, prostitutes, those not-registered with a medical practice

Question 31

What is theoretical sampling?

Answer 31

The research starts from a homogenous (small) sample and moves to a heterogenous (larger) sampling. Sampling occurs sequentially and alonside data analysis. Analysed data guides the areas to be explored further in the next round of data collection and the focus of the sample in order to achieve this.

Mostly used in grounded theory.

Question 32

What is quota sampling?

Answer 32

A non-probability sampling strategy where the researcher identifies the various strata of a population and ensures that all these strata are proportionately represented within the sample to increase its representativeness.

Question 33

What is random sampling?

Answer 33

A process of selecting a sample whereby each member of the population has an equal chance of being included.

Question 34

What are some ways to assess reliability?

Answer 34

Test-retest, split-half and alternate forms
Statistical tests such as Cronbach’s alpha and the Spearman rho correlation coefficient test

Question 35

What is reliability?

Answer 35

Reliability tells you how consistently a method measures something. When you apply the same method to the same sample under the same conditions, you should get the same results. If not, the method of measurement may be unreliable or bias may have crept into your research.

Concerned with consistency and dependability of a measuring instrument, that is, the degree to which it gives the same answer over time, across similar groups, and irrespective of who administers it.

A reliable measuring instrument will always give the same result on different occasions, assuming that what is being measured has not changed during the intervening period.