Here's how to validate data based on a cell reference to another sheet in Excel:

Method 1: Using Formula in Data Validation

1. Select the cell(s) for validation: Choose the cell(s) where you want to restrict data entry based on another sheet's value.
2. Go to Data Validation: Navigate to the "Data" tab and click on "Data Validation" in the "Data Tools" group.
3. Set Allow to Formula: In the "Settings" tab, under "Allow," choose "Formula" from the dropdown menu.
4. Enter the validation formula: In the "Formula" box, type the formula that checks the reference sheet's cell and allows data entry only if the condition is met. Here's the basic structure:

`=INDIRECT("'" & SheetName!ReferenceCell & "'")` (comparison operator) (value to compare with)

- INDIRECT: This function allows you to reference a cell based on a text string (the sheet name and cell reference). Note: Using INDIRECT can introduce potential errors and slow down calculations, so use it with caution.
- SheetName: Replace this with the actual name of the sheet containing the reference cell.
- ReferenceCell: Replace this with the cell address in the other sheet that holds the validation criteria (e.g., A1, B5).
- Comparison operator: Replace this with the desired comparison operator depending on your validation rule (e.g., ">=", "<", "<>").
- Value to compare with: Replace this with the specific value or cell reference (within the current sheet) that you want to allow or restrict based on the other sheet's value.

Example: Let's say you want to allow data entry in cell A1 only if the value in cell B1 on Sheet2 is greater than or equal to 10. The formula would be:

`=INDIRECT("'" & Sheet2!B1 & "'") >= 10`

5. Click OK: Once you've entered the formula, click "OK" to activate the data validation rule.

Method 2: Using a Named Range with Data Validation

1. Create a named range on the reference sheet (optional): If you plan to use the same reference cell for validation in multiple places, consider creating a named range for it on the reference sheet. This improves readability and maintainability.
- Go to the sheet containing the reference cell (Sheet2 in this example).
- Select the cell (B1).
- Go to the "Formulas" tab.
- Click "Define Name" in the "Defined Names" group.
- Type a clear name for the range (e.g., "ValidationCriteria").
- In the "Refers to" box, ensure cell B1 is referenced.
- Click "OK" to create the named range.
2. Select the cell(s) for validation: Similar to method 1, choose the cell(s) where you want the validation.
3. Go to Data Validation: Navigate to the "Data" tab and click "Data Validation."
4. Set Allow to Formula: In the "Settings" tab, under "Allow," choose "Formula" from the dropdown menu.
5. Enter the validation formula: Here, you can directly reference the named range instead of using INDIRECT. The formula would be:

`=Sheet2!ValidationCriteria >= 10` (assuming you named the range "ValidationCriteria")

6. Click OK: Click "OK" to activate the data validation rule.

With either method, when a user enters data into the validated cell, Excel will check the formula. If the data meets the criteria based on the reference sheet's value, it will be accepted. Otherwise, Excel will display an error message prompting the user to enter a valid value.