Many people look at the simplicity of a spreadsheet and just start typing numbers in.
However the very fact that the spreadsheet is simple and flexible means that you have to think and plan it before you start your real work. Many people think that the planning stage of any project is a waste of time when they want to get on with some tangible work. However, by properly planning your spreadsheet you can save yourself a lot of effort later and make the sheet much more readable to yourself and anyone else. Therefore every spreadsheet you produce should have the characteristics listed below. If the numbers in your spreadsheet, or the calculations you set up, are simply wrong then the results and your conclusions will be wrong.