Watch how to control the values entered into a Google Spreadsheet using data validation:
Collaboration in Google Docs is by far one of our favorite features. But we’ve noticed that one issue arises when people are collaborating in Spreadsheets. Sometimes you want to limit what people can input into a field, to make data collection easier. Data validation helps you control what data you and your collaborators enter in your spreadsheets. Here’s what you can do with data validation:
1.) Validate numbers, dates, and text.
2.) Prevent users from entering text that doesn’t match your criteria, or show them a comment on the cell when the data doesn’t match the validation rule.
3.) Add text to the cells you add validation to, to help your collaborators enter the right type of data.
Reference: Google Gooru - How to Control the Values entered into a Google Spreadsheet