Cardy

Note

This project is in alpha, significant changes and additions are expected.

Low-level card sorting utilities to compare card sorts — including calculating edit distances, d-neighbourhoods, d-cliques, and orthogonality of card sorts.

It is recommended to read Deibel et al. (2005)¹ and Fossum & Haller (2005)² to familiarize yourself with the metrics covered in this library. In fact, that entire special issue of Expert Systems is excellent reading for anyone interested in analysing card sorting data.

Installation

pip install cardy

Usage

Card sorts are represented as collections of sets of cards: Colection[Set[T]] where each set represents a group.

Edit Distance

The edit distance between two sorts can be computed with the distance function:

from cardy import distance

sort1 = ({1, 2, 3}, {4, 5, 6}, {7, 8, 9})
sort2 = ({1, 2}, {3, 4}, {5, 6, 7}, {8, 9})

dist = distance(sort1, sort2)
print("Distance:", dist)  # Distance: 3

When comparing sorts for equality, assert an edit distance of zero:

if distance(sort1, sort2) == 0:
    ...

Cliques and Neighbourhoods

Cliques and neighbourhoods can be calculated using the clique and neighbourhood functions. Given a mapping of sort IDs to card sorts: Mapping[K, Collection[Set[T]]], a neighbourhood or clique is represented as a set of IDs: Set[K] of card sorts

Neighbourhoods

Neighbourhoods are always deterministic:

from cardy import neighbourhood

probe = ({1, 2, 3, 4, 5},)
sorts = {
    0: ({1, 2, 3}, {4, 5}),
    1: ({1, 2, 3}, {4, 5}, set()),
    2: ({1, 2}, {3}, {4, 5}),
    3: ({1, 2}, {3, 4}, {5}),
    4: ({1, 2, 4}, {3, 5}),
}

two_neighbourhood = neighbourhood(2, probe, sorts)
print(f"2-neighbourhood around `{probe}`: {two_neighbourhood}")
# 2-neighbourhood around `({1, 2, 3, 4, 5},)`: {0, 1, 4}

Cliques

Cliques can be non-deterministic — even when using a greedy strategy (default):

from cardy import clique

probe = ({1, 2}, {3})
sorts = {
    0: ({1}, {2}, {3}),
    1: ({2, 3}, {1}),
    2: ({1, 2, 3},),
}
one_clique = clique(1, probe, sorts)
print(f"1-clique around `{probe}`: {one_clique}")
# 1-clique around `({1, 2}, {3})`: {0, 1}
# OR
# 1-clique around `({1, 2}, {3})`: {1, 2}

The clique function allows for various heuristic strategies for selecting candidate card sorts (via ID). Heuristic functions are of the form: (int, Mapping[K, Collection[Set[T]]]) -> K — that is, a function that takes a the maximum clique diameter and a key to card sort mapping of viable candidates, and returns a key of a viable candidate based on some heuristic.

Two heuristic functions have been provided: random_strategy and greedy_strategy. random_strategy will select a candidate at random. greedy_strategy will select a candidate that reduces the size of the candidate pool by the smallest amount. In the case two or more candidates reduce the pool by the same amount, one is selected at random.

This behaviour can be changed by providing a deterministic heuristic function, or a deterministic Selector which provides a select method that picks a candidate in the case of ambiguity:

from cardy import clique
from cardy.clique import Selector, greedy_strategy


class MinSelector(Selector):
    def select(self, collection):
        # selects the candidate with the smallest key in case of ties
        # for greedy strategy
        return min(collection)


probe = ({1, 2}, {3})
sorts = {
    0: ({1}, {2}, {3}),
    1: ({2, 3}, {1}),
    2: ({1, 2, 3},),
}
one_clique = clique(
    1,
    probe,
    sorts,
    strategy=lambda d, c: greedy_strategy(d, c, MinSelector())
)
print(f"1-clique around `{probe}`: {one_clique}")
# 1-clique around `({1, 2}, {3})`: {0, 1}

Alternatively, a seed can be passed to the base Selector constructor.

Orthogonality

The orthogonality of a collection of sorts can be calculated with the orthogonality function:

from cardy import orthogonality

p1 = (
    ({1, 3, 4, 5, 6, 7, 13, 14, 15, 22, 23},
     {2, 8, 9, 10, 11, 12, 16, 17, 18, 19, 20, 21, 24, 25, 26}),
    ({1, 3, 4, 6, 7, 10, 13, 14, 15, 18, 23, 26},
     {2, 5, 8, 9, 11, 12, 16, 17, 19, 20, 21, 22, 24, 25}),
    ({1, 2, 5, 8, 9, 11, 12, 16, 17, 18, 19, 20, 21, 22, 24, 25},
     {3, 4, 6, 7, 10, 13, 14, 15, 23, 26}),
)
p1_orthogonality = orthogonality(p1)
print(f"P1 orthogonality: {p1_orthogonality:.2f}")  # P1 orthogonality: 2.33

Footnotes

1. Deibel, K., Anderson, R. and Anderson, R. (2005), Using edit distance to analyze card sorts. Expert Systems, 22: 129-138. https://doi.org/10.1111/j.1468-0394.2005.00304.x ↩

2. Fossum, T. and Haller, S. (2005), Measuring card sort orthogonality. Expert Systems, 22: 139-146. https://doi.org/10.1111/j.1468-0394.2005.00305.x ↩