Week_7_Sampling Technique & Sample Size

Question 1

Two types of sampling technique

Answer 1

Non-probability sampling: Relies on convenience or the personal judgment of the researcher rather than chance to select sample elements. Yield good estimates of the population characteristics. Do not allow for objective evaluation of the precision of the sample results.
Probability sampling: Sampling units are selected by chance. Can determine the precision of the sample estimates of the characteristics of interest.

Question 2

What are four probability sampling methods

Answer 2

– Simple random sampling
– Systematic sampling
– Stratified sampling
– Cluster sampling

Question 3

Simple random sampling

Answer 3

A probability sampling technique in which each element in the population has a known and equal probability of selection. Every element is selected independently of every other element and the sample is drawn by a random procedure from a sampling frame.

Question 4

When can use simple random sampling

Answer 4

1. Use when population is small.

2. Use when population is large but with electronic database which can draw random sample.

Question 5

How to calculate probability of selection

Answer 5

Sample size/population size

Question 6

Procedure for drawing simple random samples

Answer 6

1. Select a suitable sampling frame.
2. Assign each element a number from 1 to N
   (population size).
3. Generate n (sample size) different random numbers
   between 1 and N. This can be done using a software
   package or using a table of simple random numbers. 4. The numbers generated denote the elements that
   should be included in the sample.

Question 7

What is random digit dialing? (RDD)

Answer 7

a method of randomly generating numbers to represent telephone numbers.
Used in telephone surveys to overcome the problem of unlisted phone numbers.

Question 8

What is plus-one dialing

Answer 8

Number drawn from the directory has the last digit in the number replaced by a random number.
Ensures that both listed and unlisted numbers are included in the sample

Question 9

Systematic Sampling

Answer 9

Chosen by selecting a random starting point and then picking every ith element in succession from the sampling frame. The sample interval is constant.
For instance: 23, 123,223,323

Question 10

Stratified Sampling

Answer 10

Population contains subgroups. Subgroup response differently to questions. Stratified sampling is appropriate when we expect each subgroup to respond to research questions DIFFERENTLY.

Question 11

The strata in stratified sampling

Answer 11

Must be mutual exclusive and collectively exhaustive.

Question 12

The mean and variance are different for each stratum

Answer 12

More flatter the distribution, greater the variance.

Small group usually has flat distribution, need to select more sample from small group

Question 13

The proportionate sample definition

Answer 13

Select sample size based on each stratum’s proportionate share of total population

Question 14

The disproportionate sample definition

Answer 14

Select sample size based on each stratum’s variance per sample size formula. which means select bigger sample size on higher variance strata, smaller sample size from lower variance strata.

Question 15

Which sample selection of stratified sampling is statistic al efficiency

Answer 15

Disproportionate sample.

Question 16

How to calculate the overall mean for the entire population by using stratified sampling?

Answer 16

Weighted average formula:

AveragePopulation = (AverageA)(ProportionA) + (AverageB)(ProportionB)

Question 17

Cluster sampling definition

Answer 17

Conduct survey by zip code.
One step area sample: if each cluster is representative, then sample from one or more group
Two step area sample: if cluster is different, sampling from different group

Question 18

Stratified sampling and cluster sampling subpopulation

Answer 18

stratified sampling need to select from each group. cluster sampling can choose one group.

Question 19

Comparison for stratified and cluster sampling

Answer 19

See slides 21

Question 20

What is sampling frame?

Answer 20

Like student ID, company code list

Question 21

Non-probability sampling plans are sometimes used

Answer 21

1. No time.

2. Managers are perfectly happy to ask n number of persons a question to help them make a decision

Question 22

Four non-probability sampling

Answer 22

– Convenience samples
– Judgment samples
– Referral samples
– Quota samples

Question 23

Quota sample definition

Answer 23

a specific quota for various types of individuals to be interviewed.

Question 24

Probability and non-probability sampling comparison

Answer 24

See slide 30

Question 25

Weakness of SRS

Answer 25

hard to construct sampling frame, lower precision, no assurance of representativeness.

Question 26

Advantage of systemic sampling

Answer 26

better than SRS because it has groups, representativeness increase.

Question 27

Disadvantage of systemic sampling

Answer 27

If cyclical patterns exist, representativeness decrease.

Question 28

Pros and cons for stratified sampling

Answer 28

Includes all important subpopulations, precision.

Difficult to select relevant stratification variables, not feasible to stratify on many variables, expensive.

Question 29

Pros and cons for cluster sampling

Answer 29

Easy to implement, cost- effective.

Imprecise, difficult to compute and interpret results

Question 30

Online Sampling Techniques

Answer 30

• Random online intercept sampling relies on a random selection of website visitors
• Invitation online sampling is when potential respondents are alerted that they may fill out a questionnaire that is hosted at a specific website
• Online panel sampling refers to consumer or other respondent panels that are set up by marketing research companies for the explicit purpose of conducting online surveys with representative samples

Question 31

Which online sampling techniques can use marketing company?

Answer 31

Online panel sampling

Question 32

Challenges for international sampling

Answer 32

Hard to get sampling frame
Hard to find decision maker
Accuracy

Question 33

Challenges for social media sampling

Answer 33

Sample not representative

Careful to choose keywords

Question 34

Ethics in sampling

Answer 34

• Appropriate definitions of the population, sampling frame, and sampling technique are essential if the research is to be conducted and the findings are to be used ethically.
• Probability sampling techniques should be used whenever the results are to be projected to the population.
• When conducting research with small populations, as in business-to-business marketing or employee research, researchers must be sensitive to preserving the respondents’ anonymity.

Question 35

Census definition

Answer 35

An accounting of everyone in the population.

Question 36

What are 3 factors affect sample size?

Answer 36

– Amount of Diversity or Variation
As diversity/variation increases, larger samples are
required
– Degree of Precision
As need for precision increases, larger samples are
required
– Degree of Confidence
Confidence increases as sample size increases

Question 37

trade-off between confidence and precision.

Answer 37

Higher precision means lower confidence unless we can increase the sample size

Question 38

What does accuracy of sample do?

Answer 38

Describe amount of sample error

Question 39

Trade off between sample size and margin of error

Answer 39

Increase sample size, decrease the margin error

Question 40

Z value

Answer 40

Standard error associated with chosen level of confidence (95% = 1.96 or 99% = 2.58)

Question 41

Sample Size Formula

Answer 41

n = z^2 (p * q)/e^2
 Where:
n= sample size
p=estimated % in population 
q=(100%-p)
e= acceptable error (accuracy) normally keep it as 5%

Question 42

Low variability definition for sample size

Answer 42

Most of people select one category (P=90%), other select the other (q=10%).
Hence, low variability gives low sample size (n)

Question 43

Hight variability definition sample size

Answer 43

In contrast to low variability

Question 44

Sample Size When Estimating a Mean

Answer 44

n = s^2*z^2/e^2
Where:
n= sample size
s=variability estimated by one standard deviation
z= Standard error associated with chosen level of confidence (1.96 or 2.58)
e= acceptable error (accuracy)

Question 45

Estimating Variability (s) in Populations

Answer 45

There are three ways to estimate variability (indicated by one standard deviation s, in the population):
1. First, do you have a previous study on the same population from which we can calculate s?
2. Second, do you have a pilot study to calculate s? …and, third…
3. When the first two choices are not available we
estimate the range of values that may be derived from the question and divide this range by 6.
(+/-3 standard deviations account for 99% of the area under the normal curve, so 6 standard deviations are synonymous with the range). By dividing the range by 6 we can estimate 1 s!