Preparation.
WHAT TO CHECK FIRST
Duplicates Missing values Obvious errors Inconsistent categories Values outside plausible ranges
WHAT DUPLICATES CAUSE
Overstated totals.
WHAT MISSING VALUES CAUSE
Understated totals, or excluded records you did not notice.
WHAT TO DO ABOUT MISSING VALUES
Establish whether the value is genuinely absent or simply unrecorded.
Those need different treatment.
WHAT TO DO ABOUT OUTLIERS
Investigate before removing.
An unusual value may be an error or the most interesting record you have.
WHAT NEVER TO DO
Delete records because they are inconvenient Alter figures to produce a tidier result
WHAT TO RECORD
What you cleaned and why.
WHY
So the analysis can be reproduced, and so you can defend it.
WHAT TO KEEP
The original, untouched.
Work on a copy, always.
HOW LONG THIS TAKES
Longer than the analysis itself. That is normal.
