You use a very large spreadsheet (or numerous spreadsheets) that holds a certain analysis of your data. You typically move columns with results around to different tabs, to further distill your results. You also copy-paste some of these columns into other sheets. You fiddle and fidget around. A few months later, you find a mistake. You make some corrections. Rinse and repeat. By the time you start writing, you fiercely try to remember all the manipulations you made to your SuperDuperSheets.
You're not alone. This may or may not have happened to me *__*
Only after I defended and finished - I came across a very smart way to keep my spreadsheets under control.
Add a "version management" tab to your spreadsheet.Now, let me expand on this idea.
Start by adding an extra "version management" tab to a new spreadsheet. In this sheet, carefully write down a version name (name of the file, typically) in the first column, in the second column the date, and in a third column an explanation of all changes you made to the sheet. Carefully fill out this sheet every single time you move something around, or tinker with the sheet.
If you're a starting PhD student, start doing this the very next time you build a new sheet. Thank me later.
If you already have multiheaded monstrous sheets: start by managing them in this way, and take a few extra hours to redefine the logic behind what you did earlier. Your dissertation-writing self will thank you.
How do you keep track of your modifications in calculation sheets?