Random sequence generation [ 23, 28, 31, 32] and allocation concealment [ 19, 23, 28, 32] were described adequately in four studies respectively.
However, only ten studies were judged to have both adequate sequence generation and allocation concealment [ 27, 33, 36, 38, 41, 44, 45, 47, 48, 51].
Although all studies were randomised controlled trials (RCTs), only 14 (50.0%) studies clearly described randomisation [ 21, 25– 37] and allocation concealment [ 21, 22, 26– 29, 31, 33– 33].
Adequate randomized sequences were generated in eight trials [ 6, 7, 17- 19, 21, 22, 26], and the investigators in five trials reported appropriate allocation concealment [ 6, 7, 19, 21, 22].
Half the studies reported adequate randomization, but only seven adequate allocation concealment [ 4, 6, 21, 47, 52, 69, 80].
Generally, risk of selection bias was high; 6 out of 12 studies did not report adequate random sequence generation and/or allocation concealment [ 23, 25, 28, 29, 31, 32].
Four of the eight trials reported adequate allocation concealment [ 29, 30, 33, 35], while in another four trials the allocation concealment was unclear.
The most common shortcomings were lack of blinding [ 47– 49, 51], lack of concealment [ 46– 49, 51], and inadequate random sequence generation [ 46– 49, 51].
Allocation concealment 4.
(a) without spatial concealment: 26.742 dB; (b) with spatial concealment: 27.868 dB.
