Qualitative and quantitative sampling

Who are theyBlack/Blue/Green/RedThin/BoldSmiling/Normal/Sad

Non-Probability Sampling

• Haphazard sampling

• Quota sampling

• Purposive sampling

• Snowball sampling

• Deviant case sampling

• Sequential sampling

• Theoretical sampling

Haphazard / Accidental / Convenience sample

Quota sample

Purposive sample

Snowball sample

Deviant case sampling

Sequential sampling(= purposive + collect cases until marginal utility drops significantly)

Theoretical sampling

• Based on (grounded) theory– Theory develops from initial research– Cases are selected that are expected to

further deepen the theory• Eg. Theory developed from data collected during

day time next collect data at night

Probability SamplingPopulation

(delinquents)N

Target population(thieves)

Sampling frame(known as

“thieves” by police)

Samplen

Sampling ratio

Nn

Probability Sampling

• Population is often (usually) unknown,therefor a population is described with theoretical values (parameters) eg: N(,)

• Random sample– Equal chance for all elements to figure in the sample

• Sampling error (deviation from representativeness)• Sampling distribution• Central limit theorem

Other demos : 1, 2

Theoretical sampling distribution

Population

Sample Sample Means Samples of 2

Cases

Samples of 4

Cases

Samples of 6

Cases .5 1 1.0 1 15 -1.75 2 2 2.0 - 2.67 5 10 2 2.75 - 3.25 3 25 10 3.33 - 4.00 8 43 52 4.17 - 4.83 5 50 82 5.00 - 5.67 8 43 52 5.75 - 6.25 3 25 10 6.33 - 7.0 5 10 2 7.25 - 7.5 2 2 8.0 1 8.5 1 No. of samples 45 210 210 Mean of s. means 4.5 4.5 4.5 % sample means > 4.00 and < 5.00

11 24 39

CASE A B C D E F G H I J Mean

SCORE 0 1 2 3 4 5 6 7 8 9 4.5

Probability sampling techniques

• Simple Random Sample (SRS)– Random numbers

• Systematic sampling (danger: cycles or patterns in sampling frame)– Random start– Sampling interval

• Stratified sampling– Subpopulating (stratification)– SRS from all strata

• Cluster sampling– Identify clusters and draw SRS from those– Then draw SRS of elements from the selected clusters– Within-Household sample: whom should researcher ask? (always first

to pick up the phone or open the door...?)– Probability Proportionate to Size (equal probabilities)

• Random Digit Dialling

A link

Simple random sample1 2 3 4 5 6 7 8 9 10

11 12 13 14 15 16 17 18 19 20

21 22 23 24 25 26 27 28 29 30

31 32 33 34 35 36 37 38 39 40

41 42 43 44 45 46 47 48 49 50

51 52 53 54 55 56 57 58 59 60

61 62 63 64 65 66 67 68 69 70

71 72 73 74 75 76 77 78 79 80

81 82 83 84 85 86 87 88 89 90

91 92 93 94 95 96 97 98 99 100

101 102 103 104 105 106 107 108 109 110

111 112 113 114 115 116 117 118 119 120

121 122 123 124 125 126 127 128 129 130

131 132 133 134 135 136 137 138 139 140

141 142 143 144 145 146 147 148 149 150

151 152 153 154 155 156 157 158 159 160

161 162 163 164 165 166 167 168 169 170

171 172 173 174 175 176 177 178 179 180

181 182 183 184 185 186 187 188 189 190

191 192 193 194 195 196 197 198 199 200

Systematic sample1 2 3 4 5 6 7 8 9 10

11 12 13 14 15 16 17 18 19 20

21 22 23 24 25 26 27 28 29 30

31 32 33 34 35 36 37 38 39 40

41 42 43 44 45 46 47 48 49 50

51 52 53 54 55 56 57 58 59 60

61 62 63 64 65 66 67 68 69 70

71 72 73 74 75 76 77 78 79 80

81 82 83 84 85 86 87 88 89 90

91 92 93 94 95 96 97 98 99 100

101 102 103 104 105 106 107 108 109 110

111 112 113 114 115 116 117 118 119 120

121 122 123 124 125 126 127 128 129 130

131 132 133 134 135 136 137 138 139 140

141 142 143 144 145 146 147 148 149 150

151 152 153 154 155 156 157 158 159 160

161 162 163 164 165 166 167 168 169 170

171 172 173 174 175 176 177 178 179 180

181 182 183 184 185 186 187 188 189 190

191 192 193 194 195 196 197 198 199 200

Stratified sampling

Cluster sampling

How large should a sample be?

• ... It depends on...– population size ,

characteristics (homogeneousless / heterogeneousmore)

– spread of the data (sd)– Purposes (descriptive?

testing? (power))

Population SampleSampling

ratio

200 171 85.5%

500 352 70.4%

1000 543 54.3%

2000 745 37.2%

5000 960 19.2%

10000 1061 10.6%

20000 1121 5.6%

50000 1160 2.3%

100000 1173 1.2%

Haphazard sampling= Accidental-, Convenience-,

Availability -

Cluster sampling

Multi stage sampling

Quota sampling

Random sampling

Snowball sampling

Stratified sampling

Systematic sampling