Linking checkbox to combo box

[greatether]

Right now for several fields I am using a combo box with vaule list consisting of "yes" and "no" as the two values. I want to use checkboxes to represent the yes and no with null as the third option. I have turned on triple state and when I select the third, green square option for the checkbox in the form, the value in the table becomes null or empty. However, when I select the check or blank options in the form, the values in the table become -1 and 0 resepectively. Is there a way to make it display yes and no instead of these numerical values?

[June7]

You can set Format property for the table field to display Yes/No.

[greatether]

You can set Format property for the table field to display Yes/No.

I prefer not to use the yes/no built in data type because it does not allow null values.