Real data is messy. Before you calculate anything, check what is in the table. A few common problems:

• Mixed units. One column in millimetres and another in centimetres, or a mix of both in one column. In 1999 NASA lost the Mars Climate Orbiter largely because one team's software gave a quantity in imperial units and the other expected metric. A table cannot warn you; you have to read the units.
• Spelling and format. "Adelie", "adelie" and "Adelie " (with a trailing space) look like three groups to a computer. Dates written as 03/04/2024 mean different days in different countries.
• Impossible values. A body mass of 0 g, a flipper of 2 000 mm, a year of 2099. Sometimes a typo, sometimes a placeholder for "unknown".
• Duplicates. The same unit entered twice, which counts it double.