Chapter 13
Sampling:
quantitative and
qualitative
CONTENTS
•
•
•
•
•
Samples and populations
Representativeness
Sample size
Weighting
Sampling for qualitative research
Samples and populations
• Population:
– Total category of subjects that is the focus of attention in a
particular research project (can be non-human)
• Sample:
– A number of subjects drawn from the population
• Two key issues:
1. What procedures must be followed to ensure that the
sample is representative of the population?
2. How large should the sample be?
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge
Representativeness
• Achieved by Random sampling:
• A systematic selection process which ensures that all
members of the population have an equal chance of
inclusion in the sample
• Designed to ensure representativeness
• An unrepresentative sample is: biased
• How is random sampling achieved in practice?
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge
Sampling for household surveys
•
Ideally
– Eg. 10 million population – sample of 1000: all 10 m.
names put in a drum and 1000 drawn.
•
In practice:
– For national/regional surveys – multi-stage sampling used
1. Select states/regions
2. Within state/region select local government areas (lga) or
constituencies/electorates
3. Within lgas or constituencies/electorates for face-to-face
interviews select streets (telephone surveys select numbers at
this point)
4. Select ‘clusters’ of 10-15 houses
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge
Sampling for telephone surveys
• Telephone numbers selected at random from
telephone directory
• For large-scale surveys: automated by ComputerAided telephone Interviewing (CATI)
• Requires access to electronic directory with
residential/business numbers identified
• No directories for mobile phones
• For household and telephone surveys: select person
in household randomly: eg person with next birthday
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge
Sampling for site/user/visitor surveys
1.
ISUM: Interviewer stationary – user mobile:
– eg. interviewing at entrance/exit
– Sample by selecting: ‘next person to pass entrance/exit point
2. USIM: User stationary – interviewer mobile
– eg. interviewing people on a beach
– Interviewers should have a set route/rules to follow – eg.
‘interview every third person/group’
3. Handouts
– Handing out questionnaires to (all) visitors for self-completion
– Not generally recommended unless closely supervised –
generally very poor response rates
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge
Sampling for street/quota surveys
• Can be used when data are available on key
characteristics of population:
– age/sex structure of a community – from Census
• Interviewing target numbers determined by
population characteristics
– Eg. if Population Census indicates 12% retired: if overall
sample size is 200: interview 24 retired people
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge
Sampling for mail surveys
• Sample from mail-out list
• 100% sample often used
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge
Sampling for complex events and destination
surveys
• Different components will conform to above
guidelines – mostly site surveys
• Problem lies in combining data from different
sources for an overall result, if required.
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge
Sampling/ random assignment for experimental
research
• Samples of subjects often ‘convenience’ samples –
eg. Students
• Assignment to control and experimental groups:
– Use of random numbers
– Online: eg. www.random.org
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge
Sample size
• Required sample size is not related to population size
(except for small populations – see later)
• Criteria:
o
o
o
The required level of precision in the results
The level of detail in the proposed analysis
The available budget
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge
Level of precision – confidence intervals
• A statistic (finding) from a sample survey is an
estimate of the population statistic
• In a randomly drawn sample the sample value
has a certain probability of being in a certain
range either side of the population value
– Eg. 95% probability of being within 2 ‘standard errors’
• See ‘Normal distribution’
– Theoretical: imagine drawing lots of samples: some would
be accurate, some not
– Discussed further in Ch. 17
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge
Normal curve
(Fig. 13.1)
NUMBER
OF SAMPLES
95%
2.5%
-4
-3
Standard errors
2.5%
-2
-1
+1
Popn Value
+2
+3
+4
Standard errors
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge
Confidence intervals (CIs) (Table 13.1: Extract)
Sample
size (N)
Percentages found from sample (‘results’)
50%
40/60%
30/70%
20/80%
10/90%
5/95% 2/98% 1/99%
Confidence intervals (CIs) + %
500
+4.4
+4.3
+4.0
+3.5
+2.6
+1.9
+1.2
+0.9
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge
Confidence intervals (CIs) (Table 13.1)
Sample
size (N)
Percentages found from sample (‘results’)
50%
40/60%
30/70%
20/80%
10/90%
5/95% 2/98% 1/99%
Confidence intervals (CIs) + %
500
+4.4
+4.3
+4.0
+3.5
+2.6
+1.9
+1.2
+0.9
So CI for 20% finding is 30% +4.0 = a range of: 26.0% to 34.0%.
CI is not related to population size.
NB.
• CI for p = CI for 100-p – eg. CI is the same for 40% and 60%
• CI for 50% is the largest in absolute terms
• This table refers to 95% probability CIs – others can be calculated – eg. 99%
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge
Confidence intervals (CIs) contd (Table 13.1)
Sample
size (N)
Percentages found from sample (‘results’)
50%
40/60%
30/70%
20/80%
10/90%
5/95% 2/98% 1/99%
Confidence intervals (CIs) + %
500
+4.4
+4.3
+4.0
+3.5
+2.6
+1.9
+1.2
+0.9
2000
+ 2.2
+2.1
+2.0
+1.7
+1.3
+1.0
+0.6
+0.4
So to halve the CI it is necessary to
increase the sample fourfold.
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge
Confidence intervals (CIs) contd (Table 13.2)
Table 13.1 can be changed to present necessary
sample size for a given CI – see Table 13.2
Percentages found from sample (‘results’)
Conf.
Interval
50%
40/60%
30/70%
20/80%
+1
9600
9216
8064
6144
+2
2400
2304
2016
+4
600
576
+8
150
144
10/90%
5/95%
1/99%
3456
1824
380
1536
864
456
*
504
384
216
114
*
126
96
53
*
*
Necessary sample sizes
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge
Suggested appendix on sample size and CIs
• See Appendix 13.1
• table indicating levels of Cis
• statement indicating that they have been
taken into account
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge
Detail of proposed analysis
Sample
size
%
CI
Range, %
Comment
Survey with sample of 200
200
Bowling
20
+5.5
14.5 – 25.5
Tennis
30
+6.3
23.7 – 36.3
Ranges
overlap
Survey with sample of 500
500
Bowling
20
+3.5
16.5 – 23.5
Tennis
30
+4.0
26.0 – 34.0
Ranges do
not overlap
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge
Budget
• Key issue: halving the CI requires 4-fold increase in sample
size
– Eg. N = 250 CI for 50% = +6.2 Survey Cost = 200 x $20 = $5000
N = 1000 CI for 50% = +3.1 Survey Cost = 1000 x $20 = $20,000
• If resources not available for adequate sample size, consider:
– Pilot/exploratory study
– Qualitative study
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge
Confidence intervals and population estimates (Table 13.3)
Item
Source
Number
Population
Census
500,000
Sample
Survey
1,000
% visiting facility/year
Survey
12%
% confidence interval
Table 13.1
+ 2.0%
Estimated no. persons
12% of 500,000
60,000
CI in no. persons
+ 2% of 500,000
+ 10,000
CI as % of persons
(10,000/60,000) x 100
+ 16.7%
Frequency of visit/year
Survey
Estimated total visits
Calc.: (12% of 500,000) x 2.5
150,000
CI in no. visits
Calc: (2% of 500,000) x 2.5
+25,000
CI in % visits
Cals.: (25,000/150,000) x 100
2.5
+16.7%
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge
Sampling for small populations
• CIs are affected by population size if population is
below about 50,000
• See Table 13.3
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge
Sample size & population size: small pop’ns (Table 13.3)
Population size
Minimum sample size to achieve CI of +5% or
+1% on a sample finding of 50%
+5%
+1%
Infinite
384
9602
5 million
384
9584
1 million
384
9511
500,000
384
9422
100,000
383
8761
50,000
381
8056
10,000
370
4899
5000
357
3288
1000
278
906
100
80
99
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge
Weighting (Tables 13.4 & 13.5)
Example: one survey at a site
Time
9-11 am
11.01-1 pm
1.01-3 pm
3.01-5 pm
Total
# of
Interviews
10
12
11
12
45
%
22.2
26.7
24.4
26.7
100.0
Actual # of
users (counts)
25
240
110
60
435
%
5.7
55.2
25.3
2.7
100.0
Sample does not reflect
the pattern of use
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge
Weighting contd (Table 13.5)
Time
Source:
9-11 am
11.01-1 pm
1.01-3 pm
3.01- 5pm
Total
A
No. of
Interviews
Survey
10
12
11
12
45
B
No. of Users
Counts
25
240
110
60
435
C
Weighting
Factors
B/A
2.5
20.0
10.0
5.0
D
Weighted
Sample No.
CxA
25
240
110
60
435
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge
Sampling for qualitative research
• Number of subjects generally be small, but:
– sampling process is still important
– should be fully described in research report
• A range of approaches is possible
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge
Sampling for qualitative research contd (Table 13.6)
Method
Convenience
Criterion
Homogeneous
Opportunistic
Maximum variation
Purposeful
Snowball
Stratified purposeful
Characteristics
Conveniently located persons or organisations Selected on key criterion - eg. age-group.
Deliberately homogeneous group: eg. universityeducated male cyclists aged 20-30.
Taking advantages of opportunities as they arise - eg. a
major sporting event taking place locally.
Deliberately studying contrasting cases. Opposite of
'homogeneous'.
Similar to 'criterion' but may involve other
considerations, such as 'maximum variation', typicality.
Interviewees source of suggestions for contacts.
A range of cases based on set criteria, eg.
representatives of a range of age-groups or nationalities.
A. J. Veal & S. Darcy (2014) Research Methods for Sport Studies and Sport Management: A practical guide. London: Routledge