- Legend: A simple factorial design (2 × 2) has 4 separate treatment arms represented by the 4 cells inside the Table (A, B, C and D). In the ATBC study [7] the four treatments were: cell A - active vitamin E with active β-carotene; cell B - active β-carotene with placebo vitamin E; cell C - active vitamin E with placebo β-carotene; and cell D - placebo vitamin E with placebo β-carotene. An ‘inside the table’ analysis uses two cells and compares active vitamin E alone (cell C) to all placebo (cell D) or compares active vitamin E alone (cell C) to active β-carotene alone (cell B); an ‘at the margins’ analysis uses 4 cells and compares all active vitamin E against and all placebo vitamin E (margin cell A + C vs margin cell B + D). The analysis used in the Cochrane review [3] uses 3 cells and compares all active vitamin E (margin cell A + C) to placebo (cell D)
