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We should learn... What is systematic sampling? It’s advantages and disadvantages... How does systematic sampling compare with SRS? What is repeated systematic sampling? Shivon Sue-Chee Systematic Sampling 1 Systematic Sampling (Ch.7) choose a random starting point and choose each 10th, or 15th, or 20th observation Eg, every 10th person leaving a shopping centre Eg, every 15th item in an ordered list of accounts Eg, every half hour/fifteen minutes/ ... take an item from an assembly line for inspection Eg, our class heights (for n=10) Shivon Sue-Chee Systematic Sampling 2 Class example example: our heights (for n=10) > N=length(height); N [1] 217 > n=10 > #Systematic random sampling > k=floor(N/n);k [1] 21 > #random starting point > set.seed(1234) > start=sample(1:k,1); start [1] 19 > sys<-seq(start,N,k);sys [1] 19 40 61 82 103 124 145 166 187 208 > sample_sys<-height[sys]; > sample_sys [1] 170.0 160.0 170.5 180.0 181.0 170.0 175.0 175.0 172.0 180.0 Shivon Sue-Chee Systematic Sampling 3 Advantages and disadvantages of systematic sampling easier, especially with no sampling frame easier to organize, especially with untrained interviewers more precise if the y ’s in the population are ordered equivalent to a SRS if the y ’s in the population are in random order hard to estimate the variance of y¯ so we usually use the SRS variance estimate very biased if the y ’s in the population are cyclical, and the sampling interval coincides with the cycle see Figures 7.1 – 7.3 Shivon Sue-Chee Systematic Sampling 4 Population types Shivon Sue-Chee Systematic Sampling 5 What are Random, Ordered and Periodic populations? Examples... Random population (ρ ≈ 0) The elements of the population are in random order. Ordered population (ρ < 0) The elements of the population have values that trend upward or downward when listed. Periodic population (ρ ≈ 1) The elements of the population tend to cycle upward and downward in a regular pattern when listed. Shivon Sue-Chee Systematic Sampling 6 Inference from systematic samples: §7.3, 7.4 comparable to SRS When systematic sampling is nearly equivalent to SRS, we can estimate V (y¯sy ) by V̂ (y¯). see formulas (7.1) and (7.2); (7.5) and (7.6); (7.7) and (7.8) HW: there’s a (small) mistake in formula (7.3) in the 7th edition: what is it? usual technique for estimating sample size n (§7.5) Otherwise, SRS variance provides useful LB or UB for V (y¯sy ). Shivon Sue-Chee Systematic Sampling 7 Inference from systematic samples: §7.3, 7.4 comparable to SRS When systematic sampling is nearly equivalent to SRS, we can estimate V (y¯sy ) by V̂ (y¯). see formulas (7.1) and (7.2); (7.5) and (7.6); (7.7) and (7.8) HW: there’s a (small) mistake in formula (7.3) in the 7th edition: what is it? usual technique for estimating sample size n (§7.5) Otherwise, SRS variance provides useful LB or UB for V (y¯sy ). Shivon Sue-Chee Systematic Sampling 8 Inference from systematic samples: §7.3, 7.4 comparable to SRS When systematic sampling is nearly equivalent to SRS, we can estimate V (y¯sy ) by V̂ (y¯). see formulas (7.1) and (7.2); (7.5) and (7.6); (7.7) and (7.8) HW: there’s a (small) mistake in formula (7.3) in the 7th edition: what is it? usual technique for estimating sample size n (§7.5) Otherwise, SRS variance provides useful LB or UB for V (y¯sy ). Shivon Sue-Chee Systematic Sampling 9 Inference from systematic samples: §7.3, 7.4 comparable to SRS When systematic sampling is nearly equivalent to SRS, we can estimate V (y¯sy ) by V̂ (y¯). see formulas (7.1) and (7.2); (7.5) and (7.6); (7.7) and (7.8) HW: there’s a (small) mistake in formula (7.3) in the 7th edition: what is it? usual technique for estimating sample size n (§7.5) Otherwise, SRS variance provides useful LB or UB for V (y¯sy ). Shivon Sue-Chee Systematic Sampling 10 Inference from systematic samples: §7.3, 7.4 comparable to SRS When systematic sampling is nearly equivalent to SRS, we can estimate V (y¯sy ) by V̂ (y¯). see formulas (7.1) and (7.2); (7.5) and (7.6); (7.7) and (7.8) HW: there’s a (small) mistake in formula (7.3) in the 7th edition: what is it? usual technique for estimating sample size n (§7.5) Otherwise, SRS variance provides useful LB or UB for V (y¯sy ). Shivon Sue-Chee Systematic Sampling 11 Inference from systematic samples: §7.3, 7.4 When are the SRS formulas okay? if the sample is ‘similar’ to a simple random sample i.e. population is unordered, with respect to the variable of interest SRS variance will be an over-estimate if the population is ordered SRS estimate will just be wrong if the population is periodic and the sampling interval coincides with the cycle e.g. data on store sales taken every 7th day; data on rainfall taken every 12 months; ... Shivon Sue-Chee Systematic Sampling 12 Inference from systematic samples: §7.3, 7.4 When are the SRS formulas okay? if the sample is ‘similar’ to a simple random sample i.e. population is unordered, with respect to the variable of interest SRS variance will be an over-estimate if the population is ordered SRS estimate will just be wrong if the population is periodic and the sampling interval coincides with the cycle e.g. data on store sales taken every 7th day; data on rainfall taken every 12 months; ... Shivon Sue-Chee Systematic Sampling 13 Inference from systematic samples: §7.3, 7.4 When are the SRS formulas okay? if the sample is ‘similar’ to a simple random sample i.e. population is unordered, with respect to the variable of interest SRS variance will be an over-estimate if the population is ordered SRS estimate will just be wrong if the population is periodic and the sampling interval coincides with the cycle e.g. data on store sales taken every 7th day; data on rainfall taken every 12 months; ... Shivon Sue-Chee Systematic Sampling 14 Why is SRS variance estimate too big if the population is ordered?