Card sorting · Analysis · Similarity matrix

How to Read a Card Sorting Similarity Matrix

By UXbeam · Information architecture tools since 2021 · Published September 21, 2026

The similarity matrix shows how often each pair of cards was grouped together. The dendrogram is built from it, and it is the view to return to when a group looks wrong. This page explains what a cell counts, how to read a row, and what a cell cannot decide.

Rows, columns, cells

Rows and columns are the cards, in the same order. The cell where a row meets a column holds the number of participant sorts in which those two cards were placed in the same group. The matrix is symmetric: the cell for Returns and Order tracking is the same as the cell for Order tracking and Returns. In UXbeam the rows are ordered by the current dendrogram grouping, so cards that were often sorted together sit near each other and a group appears as a dark square along the diagonal. Hovering a cell shows the count and the total.

The count and its denominator

The count is the number of included sorts in which both cards shared a group. The total shown beside it is the number of included participant sorts, all of them. A participant who left one of the two cards unsorted is in the total and not in the count, so an unsorted card contributes no co-sort count for that pair, even though that participant did not place the cards in separate groups. Excluded participants are in neither. The shade of a cell is the count as a share of the total.

The diagonal, each card with itself, is set to the total. It anchors the eye and carries no evidence about placement.

Example matrix

Each cell: the number of sorts, out of twelve, in which the two cards shared a group. Darker is more. The diagonal is the sort total.

A worked reading

Read a row, not a cell. The Order tracking row says ten of twelve sorts put it with Shipping options, six with Returns, and two or fewer with anything else. That is a card with one strong partner and one moderate one.

The Returns row is the interesting one: six with Order tracking, six with Customer support, five with Shipping options. Half the sorts put it with orders and half with support, and they are largely different halves: only two sorts put all three cards together. Returns has two candidate placements to investigate. The listed sorts show that; it does not say which home to choose.

Payment methods and Gift cards share seven sorts and almost nothing with anyone else. A pair with no third member is a small group, or a sign that the set is missing the cards that would make it one.

Checking other explanations

Shipping options has ten with Order tracking and three with Payment methods. Two readings: a quarter of the sample meets shipping choices at checkout and grouped it with paying, or the card name made three participants think of shipping costs. The matrix cannot separate a placement instinct from a wording effect. Inspect those participants' labels and groupings, then compare candidate placements with a task about choosing delivery speed.

Blocks and exceptions

A block is a set of cards that are all high with each other: Order tracking and Shipping options above, or the two payment cards. Blocks are what the dendrogram turns into groups: the dendrogram page draws the tree built from exactly these twelve sorts. An exception is a cell that breaks the pattern: a high count outside a block, which marks a bridge card like Returns, or a low count inside one, which marks a pair within the displayed group that participants rarely sorted together. Reordering the rows changes what is easy to see and changes no number.

Caveats

• Unsorted cards contribute no co-sort count. If a study allowed unsorted cards, a low row may reflect cards left unsorted, not that they separated it from everything.
• Small samples move in big steps. With twelve sorts, one participant is a twelfth of the shade. Do not treat small count differences as decisive without inspecting the underlying sorts.
• The matrix pools everyone. The same pair count can occur within different participant mental models. The matrix and the dendrogram are both built from the pooled sorts; Mental Model Detection identifies recurring ways participants organized the cards. Use it to investigate different mental models that a pooled similarity matrix or dendrogram may obscure.

Not a placement table

In a closed sort, the natural table is card by category: how many participants put Returns under Orders & delivery versus Account & support. That counts placements into names you supplied. The similarity matrix is card by card and counts pairs regardless of what any group was called. A closed sort has both views; an open sort also provides individual arrangements, a dendrogram and any detected mental models.

What a cell can and cannot support