Grouping and Summarizing

Grouping is a dictionary. Walk the rows, use the grouping value as the key, and accumulate.

Report the group sizes. A mean over three rows is not a finding, and presented without its count it looks exactly like a mean over three thousand.

Three things that mislead quietly.

A mean is pulled by outliers; a median is not. Which one is right depends on the question, and choosing the one that gives a better headline is the definition of misleading.

Percentages need their denominator stated. 'Up 200%' from one to three is true and useless.

Counting rows is not counting people if one person can appear more than once. Decide which you are counting and say so.