Data Validation is extremely useful functionality and I use it in every system I write.

The primary aim is to guide people into entering correct data. If you allow people to type whatever they like into a field, you end up with all kind of gunk.

There are a few ways to use Data Validation... we'll discuss the way based on other cells.

Suppose you want to restrict the number entered in the following example to even numbers between 2 and 10

Step1
Select the cell, In Excel 2003, go: Data=> Validation... In Excel 2007, Data => Data Validation and configure like so:

Step2
Click on the cell next to “Number” and press ALT + DOWN ARROW... You will now see the list appear and you can use the UP and DOWN arrows to select the one you want.

Sheet to practise with

Video Training on Data Validation: