In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: The prevalence of postoperative delirium was significantly lower in the light sedation group ( 11 / 57 [ 19 % ] vs 23 / 57 [ 40 % ] in the deep sedation group ; P = . 02 ) , indicating that 1 incident of delirium will be prevented for every 4 . 7 patients treated with light sedation .
### Output Text:
The prevalence of postoperative delirium was significantly lower in the light sedation group ( 11 / 57 [ 19 % ] vs 23 / 57 [ 40 % ] in the deep sedation group ; P = . 02 ) , indicating that 1 incident of delirium will be prevented for every 4 . 7 patients treated with light sedation . 