Validate by Calculation

[millmaine]

We have a table of Items. Each Item has a Category field. Each Item has a Size field. Depending on the Category, the Size may or may not be required. I am trying to use "Validate by Calculation" on the Size filed so that an alert is shown if it is empty when the Category requires that it should be completed but no alert is shown if the Category does not require it. So far I have had no success. I'd welcome any tips.

[millmaine]

All good. Figured it out using Case.