Validation Help

Hi, I have got a problem, and everything I have tried has not worked. I have four groups of variables that are passed from a checkbox on one page to the next. These are configured in a way so the variables are passed with the following names. p101, p102, p103... etc p201, p202, p203... etc p301, p302, p303... etc p401, p402, p403... etc Numbers one and two are compulsory, while numbers three and four are optional. Currently if no options are selected from either number one or number two the user can still continue, but we do not want them to be able to do this. We want them to select at least one checkbox from each group. Obviously if the user selects one for number two, but zero from number one the url will look something like file.php?p201=xxxx. There will be no reference to p1xx in there, so there is no way I know that I can do a if statement to check to see whether a value is assigned on the proceeding screen. Does anyone know how I can force the user to select at least one from each of the active options? Thanks, Michael