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. 2020 Mar 25;48(2):0300060520903209. doi: 10.1177/0300060520903209

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Figure 6. — Funnel plot for publication bias regarding the effect of a helmet on partial pressure of carbon dioxide (PaCO₂). Each point represents a separate study included in the meta-analysis. Squares indicate studies assessing the effects of a helmet compared with standard oxygen therapy. Diamonds indicate studies assessing the effects of a helmet compared with a mask. SE, standard error; MD, mean difference.