**Association measures** are mathematical formulae that interpret cooccurrence frequency data.
For each pair of words extracted from a corpus, they compute an **association score**, a single
real value g that indicates the amount of (statistical) association between the two words.
Some measures distinguish between *positive* (g > 0) and *negative* (g < 0) association.
Many association measures are based on statistical hypothesis tests, while some others are purely heuristic
combinations of the observed joint and marginal frequencies. In general, the association scores
computed by different measures cannot be compared directly. They are typically used to **rank**
pair types (as candidates for **collocations**). Further processing and comparison of measures is
then based on **n-best lists**, regardless of the precise association scores of the candidates.
A more detailed explanation of this approach to collocation extraction can be found in
Evert & Krenn (2001) and Evert & Krenn (2003).

These pages provide a repository of the numerous association measures that have been suggested and used over the past decades. For each measure, an explicit equation is given in terms of observed and expected frequencies (see below), which can easily be translated into a computer program (while the equations in the original publications are often cryptic or incomplete). In addition, the text provides some background information and references (when available).

Section 1 introduces cooccurrence frequency data, some notation, and a statistical random sample model on which many association measures are based. The individual measures are grouped according to their theoretical motivation and are presented in Sections 2 through 8. This page provides a table of contents with direct links to the sections. A list of association measures is given for each section, so it is easy to locate information about a specific measure.

Implementations of all association measures in this repository are available in the
**UCS toolkit**, which can be downloaded from the
software page.

Cooccurrence frequency data for a word pair (u,v) are often organised
in a **contingency table**, which results from a cross-classification of
the **pair tokens** (= instances of cooccurrences) extracted from
a corpus. Tokens whose first component belongs to type u are
assigned to the first row of the table, and tokens whose second component
belongs to type v are assigned to the first column. The cell counts of
this contingency table are called the **observed frequencies**
O_{11}, ..., O_{22} (see the right panel below).

The sum of all four observed frequencies (called the sample size N) is equal to
the total number of pair tokens extracted from the corpus.
R_{1} and R_{2} are the row totals of the observed contingency table,
while C_{1} and C_{2} are the corresponding column totals.
The row and column totals are also called **marginal frequencies**, being written in the margins
of the table, and O_{11} is called the **joint frequency**. Equations for
all association measures are given in terms of the observed frequencies, marginal frequencies, and
the **expected frequencies** E_{11}, ..., E_{22}
(under the null hypothesis that u and v are statistically independent). The
expected frequencies can easily be computed from the row and column totals as shown in the left
panel above.

Stefan Evert Last Modified: Tue Apr 6 00:10:29 2004 (evert)