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Table 3 Power calculations for intermediate and long-term outcomes

From: Determining the impact of smoking point of sale legislation among youth (Display) study: a protocol for an evaluation of public health policy

    Sample size required at 0.80 power with two-tailed test Sample before and 1 year after POS Supermarket (4 S2 & 4 S4 from 4 schools 24 pupils each class 85% response rate) Sample before Small Shops POS and 2 years after (all pupils attending schools, 4 schools, school size 1200 pupils, response rate 85%)
  Outcome Change p < .05 p < .01 4 schools combined Urbanisation (2 levels) 4 schools combined Urbanisation (2 levels)
       Deprivation (2 levels)   Deprivation (2 levels)
       Age (2 levels)   
All (community population)     N = 1280 N = 640 N = 4074 N = 2037
  Access to tobacco: If try buy, likely to be successful1 32%-25% 680 999 >0.80 0.80 >0.99 >0.99
  Access to tobacco: If try buy, likely to be successful2 32%-22% 328 479 >0.99 >0.80 >0.99 >0.99
  Awareness tobacco marketing in shops1 81%-22% 14 19 >0.99 >0.99 >0.99 >0.99
  Awareness tobacco marketing in shops2 81%-71% 305 445 >0.99 >0.99 >0.99 >0.99
  Perceived prevalence regular smoking1 62%-46% 164 238 >0.99 >0.99 >0.99 > 0.99
  Perceived prevalence regular smoking2 62%-52% 404 591 >0.99 >0.80 >0.99 >0.99
  Incidence regular smoking3 4%-2% 1239 1797 >0.80   >0.99 >0.80
  Incidence any smoking3 9%-7% 2987 4396    >0.80*  
  Prevalence regular smoking3 13%-10% 1841 2707    >0.99 >0.80
  Prevalence regular smoking3 13%-9% 1009 1478 >0.80   >0.99 >0.80
Current smokers     N = 136 N = 68 N = 482 N = 241
  Purchase tobacco from shops4 55%-43% 412 603    >0.80 0.82*
  Purchase from Supermarkets4 12%-5% 389 563    >0.80 0.85*
  Purchase from Small Shops4 (increase after POS Supermarkets 44%-56% 411 603    >0.80 0.82*
  Purchase from Small Shops4 44%-32% 393 575    >0.80 0.84*
  Access to tobacco: If try buy, likely to be successful 32%-20% 408 596    >0.80 0.83
  Awareness tobacco marketing in shops1 81%-22% 14 19 >0.99 >0.99 >0.99 0.99
  Awareness tobacco marketing in shops2 81%-71% 305 445    >0.80 0.80*
  Perceived prevalence regular smoking2 62%-50% 404 591    0 0.83*
Ever smokers     N = 422 N = 212 N = 1287 N = 644
  Purchase tobacco from shops4 23%-16% 530 776 >0.80*   >0.99 0.87
  Purchase from Supermarkets4 4%-1% 489 697 >0.80*   >0.99 0.85*
  Purchase from Small Shops4 (increase after POS Supermarkets ) 18%-25% 568 832 >0.80*   >0.99 0.85
  Purchase from Small Shops4 18%-12% 588 859 >0.80*   >0.80 >0.80*
  Access to tobacco: If try buy, likely to be successful2 32%-22% 408 596 >0.80   >0.99 >0.80
  Awareness tobacco marketing in shops1 81%-22% 14 19 >0.99 >0.99 >0.99 >0.99
  Awareness tobacco marketing in shops2 81%-71% 305 445 >0.80   >0.99 >0.99
  Perceived prevalence regular smoking2 62%-52% 404 591 >0.80   >0.99 >0.80
  1. 1 Change based on findings McNeil et al. [14] paper on the removal of Point of Sale tobacco displays in Ireland.
  2. 2 Starting percentage based on findings McNeil et al. [14] paper on the removal of Point of Sale tobacco displays in Ireland.
  3. 3 Starting percentages based on findings latest published results from HBSC [19] and SALSUS [18] studies.
  4. 4 Starting percentages based on findings latest published results from SALSUS [18].
  5. * Denotes power calculation for a one-tailed test. Blank cells are underpowered to detect a significant change.