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: However , no study in the literature has reported the use of a statistical model to analyze the variables that influence outcome .
### Output Text:
However , no study in the literature has reported the use of a statistical model to analyze the variables that influence outcome . 