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: Odds ratios were calculated using a conditional logistic model , including potential confounding factors , both for the whole study population and for the various underlying diseases .
### Output Text:
Odds ratios were calculated using a conditional logistic model , including potential confounding factors , both for the whole study population and for the various underlying diseases . 