Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
What is the highest sample size administered on October 18?

### Inputs:
table_name: table, columns: ['Poll Source', 'Date(s) administered', 'Sample size', 'Margin of Error', 'Mel Martinez (R)', 'Betty Castor (D)']

### Response:
SELECT MAX Sample size FROM table WHERE Date(s) administered = october 18