Dear All,

I need to get the value into form field which equal to records in same form. I want to crate relationship between record in form.

can I write the formula in Derivation Formula like this;

If( Lookup PIGR TAXYEAR =  TAXYEAR - 1 , TAXRATE , 0)

it give unknown relationship in form. 

Thank you in advance

It's nay on impossible to tell what is wrong here without having the application. Is there a relationship called PIGR with a Field called TAXYEAR? How to know? 

There is a "million" ways of getting this wrong so to tell you from this is sadly impossible.

from the general point of view to avoid unlimited cycling it is better to use subforms and the Relationships table