When referencing another element from the form in a formula field, there are certain syntax rules that you should take into consideration. A fixed value is any value for an element that is literal

Reference a Fixed Value

Whenever you refer to a fixed value as an element in a formula, you can use single or double quotes. 

Reference a Boolean Value

Whenever you refer to a Boolean Value for a Boolean Element in a formula, you should not use any quotation marks. This is because the only possible values for a Boolean Element are true or false and need to be evaluated, rather than taken for their literal value of the words 'true' and 'false'.