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: METHODS : Data were analyzed from four similarly designed , randomized , double - blind , 3 - to 12 - week studies .
### Output Text:
METHODS : Data were analyzed from four similarly designed , randomized , double - blind , 3 - to 12 - week studies . 