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: Also the frequency of abnormal electro - diagnostic findings showed similarity between the two groups ( G : 7 / 23 = 30 . 4 % ; P : 6 / 20 = 30 % ) .
### Output Text:
Also the frequency of abnormal electro - diagnostic findings showed similarity between the two groups ( G : 7 / 23 = 30 . 4 % ; P : 6 / 20 = 30 % ) . 