Relative frequency
relative frequency = count in category / n
Excel: =COUNTIF(range, category) / COUNTA(range)
Turning a raw categorical column into a distribution that can be charted. The relative frequencies must sum to 1.
- count in category
- Number of observations with that value
- n
- Total number of observations, not the number of categories
Row and column percentage in a cross-tab
row % = cell / row total
column % = cell / column total
Reading a cross-tabulation. Decide first which conditional question you are asking; the two percentages answer different questions and rarely agree.
- cell
- Count in one row-column combination
- row total, column total
- Margin totals of the cross-tab
Equal-interval class width for a choropleth
class width = (max - min) / number of classes
Choosing shading bands for a map. Compare against quantile bands before publishing: the same data looks calm or alarming depending on the choice.
- max, min
- Largest and smallest values in the variable
- number of classes
- How many shading bands the legend will have