Validate a group of fields so that form can only be submitted if 0 or 2+ fields have been completed In Drupal 7 I need to validate several cck fields so that the node can only be saved if 0 or two or more fields have been completed. So if only one of the fields is completed validation would fail with an error message like 'Please submit two or more years worth of data, or no data at all'.

I know that I can useField Group and Field Group Multiple To set up groups of fields. I can see that Field Validation provides basic validation options, including checking that a minimum of 1 of several fields have been completed. But as far as I can see it does not have the capacity to achieve what I need it to do.

Could you please suggest how to go about forcing a user to either complete 2+ fields or none at all?

Thanks
First I assume this must be for Drupal 6 since you are describing CCK fields? I would suggest creating a small custom module, and implementing hook_validate. 

You can then check how many fields have been entered by your user to see if it passes your requirements described above. 

If the validation fails you can then display the error to the logged in user with form_set_error

EDIT:

Since you're using Drupal 7, you can use hook_node_validate within a custom module.