Data validation and conditional formatting are powerful tools in Excel that can work together to improve data accuracy and provide visual cues. Here's how to combine them:

Scenario: Imagine you have a cell where users can select a department name from a dropdown list (data validation). You want to highlight cells with invalid department names (not on the list) with a red background (conditional formatting).

Steps:

1. Set up Data Validation (Dropdown List):
- Select the cell where you want the dropdown list.
- Go to the "Data" tab and click "Data Validation" in the "Data Tools" group.
- Choose "Allow" as "List" in the "Settings" tab.
- Define the "Source" by entering the list of valid department names separated by commas or referencing the range containing your list.

2. Create Conditional Formatting Rule:
- Select the range of cells where the dropdown list is applied (or the single cell if only using one).
- Go to the "Home" tab and click "Conditional Formatting" in the "Styles" group.
- Choose "New Rule..." from the dropdown menu.
- Select "Use a formula to determine which cells to format" under "Select a Rule Type."

3. Formula for Conditional Formatting:
- In the formula box, enter a formula that checks if the selected value in the cell is valid based on the data validation list. Here's the formula:

`=ISNA(MATCH(A1, $B$2:$B$10, 0))`

- Replace:
- A1: This is the cell reference where the dropdown list is applied. Adjust it for your specific cell location.
- $B$2:$B$10: This is the range containing your valid department name list. Adjust the range to match your actual list location.

4. Explanation of the Formula:
- `MATCH(A1, $B$2:$B$10, 0)`: This part searches for the value in cell A1 (where the dropdown is) within the range B2:B10 (your department list). The 0 specifies an exact match.
- `ISNA()`: This function checks if the result of the `MATCH` function is an error (#N/A, meaning no match found).

5. Formatting for Invalid Values:
- If the formula evaluates to TRUE (no match found), the cell value is considered invalid. Click on the "Format..." button to choose the desired formatting for invalid entries (e.g., red background).

6. Click OK: Click "OK" on all open dialog boxes to complete the rule creation.

Now, whenever someone selects a value not on the department list, the cell will be highlighted with the red background you defined. This helps identify potential errors and ensures data consistency.

Additional Tips:

- You can use this approach with other data validation types like numbers or dates. Just adjust the formula accordingly.
- Conditional formatting can be used with other criteria besides data validation results. Explore various formatting rules for a more informative spreadsheet.