Qualitative and quantitative sampling
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Qualitative and quantitative sampling Who are they Black/Blue/Green/Red Thin/Bold Smiling/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 Sampling Target population (thieves) Sampling frame (known as “thieves” by police) Sample n Sampling ratio n N Population (delinquents) N 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 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 Sample Means Samples of 2 Cases .5 1 1.0 1 15 -1.75 2 2.0 - 2.67 5 2.75 - 3.25 3 3.33 - 4.00 8 4.17 - 4.83 5 5.00 - 5.67 8 5.75 - 6.25 3 6.33 - 7.0 5 7.25 - 7.5 2 8.0 1 8.5 1 No. of samples 45 Mean of s. means 4.5 % sample means 11 > 4.00 and < 5.00 Samples of 4 Cases Samples of 6 Cases 2 10 25 43 50 43 25 10 2 2 10 52 82 52 10 2 210 4.5 24 210 4.5 39 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 sample 1 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 sample 1 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 Sample Sampling 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 Random sampling = Accidental-, Convenience-, Availability - Cluster sampling Snowball sampling Multi stage sampling Stratified sampling Quota sampling Systematic sampling
