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Table 1 The frequency of randomized studies, use of blinding and statistical significance among different income groups in the years1993 and 2003 (numbers in the parentheses represent the 95% CIs of the absolute change)

From: Country development and manuscript selection bias: a review of published studies

  Low Lower-middle Upper-middle High
Randomized studies:
1993 38/50(76%) 36/50(72%) 34/50(68%) 31/50(62%)
2003 44/50(88%) 38/50(76%) 39/50(78%) 37/50(74%)
Absolute change (%) 12 (-2 to 27) 4(-13 to 21) 10(-7 to 27) 12(-6 to 30)
Blinded studies:
1993 18/50(36%) 23/50(46%) 25/50(50%) 30/50(60%)
2003 23/50(46%) 16/50(32%) 22/50(44%) 23/50(46%)
Absolute change (%) 10(-9 to 29) -14(-32 to 5) -6(-25 to 14) -14(-33 to 5)
Significant results:
1993 29/50(58%) 30/50(60%) 38/50(76%) 34/50(68%)
2003 40/50(80%) 41/50(82%) 42/50(84%) 33/50(66%)
Absolute change (%) 22(4 to 39) 22(5 to 39) 8(-7 to 24) -2(-20 to 16)