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The Swap-Sort

Quality Tools > Tools of the Trade > 17: The Swap-Sort


In an earlier article, I introduced the Post-Up, a simple data-collection or brainstorming tool from my book, 'Rapid Problem-Solving with Post-it® Notes'. This month, I will continue this theme with a complementary Post-it® Note tool, the Swap-Sort, a technique derived from the computer bubble-sort algorithm.


How does it work?

The results of a Post-Up or any other collection of information is good in that you now have a lot of potentially useful information, but bad in that you probably do not know which are the useful pieces. The Swap-Sort provides a simple and efficient method of prioritising a list of  items. As a simple example,



The Swap-Sort

The way the Post-Up works is simply by comparing pairs of Post-it® Notes and swapping them if they are in the wrong order. This is repeated until the list is in the correct order, as illustrated below.



The sequence of swaps

The one drawback of the Swap-Sort is that it is most effective for short lists; the number of swaps that may be required in longer lists increases geometrically, as in the table below. In practice, this is usually not a large problem and a spot of list-reduction beforehand will quickly build a shortlist of items for closer attention.


Number of items in list

Maximum number of swaps













How do you do it?

1. On a flipchart, write down the objective of the session. This may typically be to sort a list into priority order or to select a few items from a longer list.


Also identify a short list of the criteria to be used in prioritising the target list. Keep this list short: from 1 to 4 criteria is ideal. You can use a mini-Swap-Sort to put these in order of importance.



Identifying criteria


2. Generate the list of items to be prioritised, each item on a separate Post-it® Note. This will typically be available from a Post-Up that you have recently done. Post these up on a flip-chart page (or two) which has been taped to the wall.


3. If there are more than about ten Post-it® Notes, reduce the list to ten or less by selecting obvious candidates, combining others and rejecting those which are obviously not candidates. It might take a few passes through this process to end up with a shortlist.


List reduction is much like finding a house: You start with a big pile, then sort out 'possibles', 'definitely nots' and duplicates. The process is then repeated until you have a smaller list to which you can apply more detailed criteria.



List reduction


4. Post the remaining list of Post-it® Notes in a vertical column on a flipchart page or whiteboard.


5. Compare the top two Post-it® Notes, using the pre-defined criteria. Are they in the right order? If not, swap them, so the most important item is at the top.


6. Repeat this process for the second two items in the list (this will include one of the items you just considered.


7. Keep repeating this process for each pair of Post-it® Notes until you have got to the bottom of the list.



Swapping Notes

8. Keep repeating this process from the top of the list, comparing and swapping, until you can go from top to the bottom of the list without swapping any Post-it® Notes. You will now have completed the activity and your list will be in priority order.


(Post-it® Notes is a registered trademark of 3M Corporation)


Next time: Z-Charts


This article first appeared in Quality World, the journal of the Institute for Quality Assurance

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