Mapping principles: the four jobs of a crosswalk

A map for a purpose

A map is always for a purpose. WHO describes crosswalks by direction and by kind.

Forward and backward

A forward crosswalk takes codes from an earlier classification to the later one, to group old data with new software and compare mortality or service use over time. A backward crosswalk takes current codes back to earlier ones, to use a previous grouper or to make a time series.

Historical and logical

Historical crosswalks select the map that gives the most appropriate coding and clinical meaning, and that may consist of one or more codes. They suit large-scale use but should be assessed case by case for specific subsets. Logical, or special-purpose, crosswalks serve casemix: each code goes to the most clinically similar code that gives the right DRG.

Match the map to the question

So match the map to the question. Comparing mortality or service use over time points to a historical map. Grouping for casemix points to a logical one.

Check yourself

Pause and think. Which kind of crosswalk serves casemix, sending each code to the most clinically similar code that gives the right DRG? The answer: A logical, or special-purpose, crosswalk. Logical, or special-purpose, crosswalks serve casemix. Historical crosswalks select the map that gives the most appropriate coding and clinical meaning.

• A logical, or special-purpose, crosswalk
• A historical crosswalk
• A forward crosswalk
• A backward crosswalk

Keep these in mind

Forward maps old to new, backward maps new to old. Historical is for meaning, logical is for casemix. And assess maps for the data you will use them on.

• Forward maps old to new; backward maps new to old
• Historical for meaning; logical for casemix
• Assess maps for the data you will use them on