position - Would putting a submit button at the top of a long form be usable?

I have a form with 3 long columns of checkboxes where the user needs to check whatever options she/he needs. Sortening the amount of checkboxes is not an option Im afraid.

Our users are mostly in the upper age groups with only basic computer experience.

I'm afraid they might check the options they want and not see the submit button at the end of the long form and just leave the page and their info not get saved.

I'm thinking of putting the submit button at the top. I also thought of checking if the user is leaving the page without having saved changes and showing a modal dialogue saying "You have unsaved changes, are you sure you want to leave the page?", etc.

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Thanks for all your great answers folks. Every solution seems to have tradeoffs. We decided to use a modal dialog at the moment and see how our users react.

We dont really have any proof that they wont see the submit button at the bottom. Nielsen says 1-2 pagefulls is OK for long forms:

Note that the sheer length of a form is not a sufficient reason to break it up. Yes, it's best to stick to one or two screenfulls: If a form takes up many screens, you should seriously consider eliminating some of the questions and options. Otherwise, you're likely to overwhelm users and reduce your conversion rate. But, if all elements are truly needed, you can certainly place them on one long page -- just ensure that each element is easy and that all users have to do is keep scrolling (and sighing) as they complete each step in the long march to the blessed submit button.

Answer

Considering your target user group, I'd use the modal dialogue in case they didn't save before closing the window.

Duplicated submit buttons can be very useful when a form can be filled partially and submitted half-way through, but this does not seem to be your case, and less tech-savvy users may become confused when they see several identical buttons on the page.

P.S.: In case this a form filled by logged-in users, have you considering auto-saving the responses? Although I'd still include a clear submit button for those who aren't used to this...