# Cramer’s V, Phi, and association measures for contingency tables in R

    library(spicy)

spicy provides a full suite of effect size and association measures for contingency tables, covering both nominal and ordinal variables. This vignette explains which measure to use depending on the measurement level of your variables, and how to obtain confidence intervals and p-values for chi-squared-based and rank-based statistics.

## Choosing the right measure

The table below summarizes the recommended measures by variable type.

Variable types | Recommended measure | Function
--- | --- | ---
Nominal x Nominal | Cramer’s V | `cramer_v()`
Nominal x Nominal | Contingency Coefficient | `contingency_coef()`
Nominal x Nominal (2x2) | Phi | `phi()`
Ordinal x Ordinal | Kendall’s Tau-b | `kendall_tau_b()`
Ordinal x Ordinal (rectangular) | Kendall’s Tau-c | `kendall_tau_c()`
Ordinal x Ordinal | Goodman-Kruskal Gamma | `gamma_gk()`
Ordinal x Ordinal (asymmetric) | Somers’ D | `somers_d()`
Nominal (asymmetric, PRE) | Lambda | `lambda_gk()`
Nominal (asymmetric, PRE) | Goodman-Kruskal Tau | `goodman_kruskal_tau()`
Nominal (asymmetric, PRE) | Uncertainty Coefficient | `uncertainty_coef()`
2x2 table | Yule’s Q | `yule_q()`

PRE = Proportional Reduction in Error. These measures quantify how much knowing one variable reduces prediction error for the other.

All functions accept a contingency table (class `table`, typically from `xtabs()` or `table()`).

## Quick overview with assoc_measures()

`assoc_measures()` computes all available measures at once:

    tbl <- xtabs(~ smoking + education, data = sochealth)
    assoc_measures(tbl)
    #> Measure                            Estimate     SE  CI lower  CI upper      p
    #> Cramer's V                            0.136     --     0.079     0.191  <.001
    #> Contingency Coefficient               0.134     --        --        --  <.001
    #> Lambda symmetric                      0.000  0.000     0.000     0.000     --
    #> Lambda R|C                            0.000  0.000     0.000     0.000     --
    #> Lambda C|R                            0.000  0.000     0.000     0.000     --
    #> Goodman-Kruskal's Tau R|C             0.018  0.008     0.003     0.034   .023
    #> Goodman-Kruskal's Tau C|R             0.008  0.003     0.001     0.014   .022
    #> Uncertainty Coefficient symmetric     0.011  0.005     0.002     0.021   .021
    #> Uncertainty Coefficient R|C           0.018  0.008     0.003     0.032   .021
    #> Uncertainty Coefficient C|R           0.009  0.004     0.001     0.016   .021
    #> Goodman-Kruskal Gamma                -0.268  0.056    -0.378    -0.158  <.001
    #> Kendall's Tau-b                      -0.126  0.027    -0.180    -0.073  <.001
    #> Kendall's Tau-c                      -0.117  0.026    -0.167    -0.067  <.001
    #> Somers' D R|C                        -0.091  0.020    -0.131    -0.052  <.001
    #> Somers' D C|R                        -0.175  0.038    -0.249    -0.101  <.001

This is useful for exploratory analysis. For reporting, pick the measure that matches your variable types.

## Nominal variables

### Cramer’s V

Cramer’s V measures the strength of association between two nominal variables. It ranges from 0 (no association) to 1 (perfect association).

    tbl <- xtabs(~ smoking + education, data = sochealth)
    cramer_v(tbl)
    #> [1] 0.1356677

Pass `detail = TRUE` for the confidence interval and p-value. The p-value tests the null hypothesis of no association using the Pearson chi-squared test.

    cramer_v(tbl, detail = TRUE)
    #> Estimate  CI lower  CI upper      p
    #>    0.136     0.079     0.191  <.001

### Phi coefficient

For 2x2 tables, Phi is equivalent to Cramer’s V. Unlike V, Phi can be negative when the table is 2x2, indicating the direction of association. The p-value tests H0: no association (Pearson chi-squared test).

    tbl_22 <- xtabs(~ smoking + physical_activity, data = sochealth)
    phi(tbl_22, detail = TRUE)
    #> Estimate  CI lower  CI upper     p
    #>    0.006     0.000     0.063  .839

### Contingency coefficient

The contingency coefficient is an alternative to Cramer’s V. Its upper bound depends on the table dimensions, which makes it harder to compare across tables of different sizes. The p-value tests H0: no association (Pearson chi-squared test).

    contingency_coef(tbl, detail = TRUE)
    #> Estimate  CI lower  CI upper      p
    #>    0.134        --        --  <.001

## Ordinal variables

When both variables are ordinal (ordered factors), measures that account for the ordering are more appropriate than Cramer’s V.

### Goodman-Kruskal Gamma

Gamma ranges from -1 to +1. It ignores tied pairs, which makes it sensitive to the direction of association but tends to overestimate strength when there are many ties.

    tbl_ord <- xtabs(~ self_rated_health + education, data = sochealth)
    gamma_gk(tbl_ord, detail = TRUE)
    #> Estimate  CI lower  CI upper      p
    #>    0.310     0.238     0.383  <.001

A positive value means that higher values on one variable tend to occur with higher values on the other. The p-value tests H0: Gamma = 0 using a Wald z-test.

### Kendall’s Tau-b

Tau-b adjusts for ties and ranges from -1 to +1. It is generally preferred over Gamma for square or near-square tables. The p-value tests H0: Tau-b = 0 (Wald z-test).

    kendall_tau_b(tbl_ord, detail = TRUE)
    #> Estimate  CI lower  CI upper      p
    #>    0.205     0.155     0.254  <.001

### Kendall’s Tau-c

Tau-c is similar to Tau-b but adjusts for rectangular tables where the number of rows and columns differ. The p-value tests H0: Tau-c = 0 (Wald z-test).

    kendall_tau_c(tbl_ord, detail = TRUE)
    #> Estimate  CI lower  CI upper      p
    #>    0.200     0.151     0.248  <.001

### Somers’ D

Somers’ D is an asymmetric measure: it distinguishes between a dependent and an independent variable. By default, the row variable is treated as dependent (D(R|C)). The p-value tests H0: D = 0 (Wald z-test).

    somers_d(tbl_ord, detail = TRUE)
    #> Estimate  CI lower  CI upper      p
    #>    0.208     0.157     0.258  <.001

## Asymmetric (PRE) measures

These measures answer a specific question: how much does knowing the column variable reduce our error in predicting the row variable (or vice versa)?

### Lambda

Lambda measures the proportional reduction in classification error. It can equal zero even when the variables are associated, if the modal category does not change across columns. The p-value tests H0: Lambda = 0 (Wald z-test).

    tbl <- xtabs(~ self_rated_health + education, data = sochealth)
    lambda_gk(tbl, detail = TRUE)
    #> Estimate  CI lower  CI upper     p
    #>    0.012     0.000     0.039  .389

### Goodman-Kruskal Tau

Tau measures the proportional reduction in error when predicting the row variable from the column variable, using the full distribution (not just the mode). The p-value tests H0: Tau = 0 (Wald z-test).

    goodman_kruskal_tau(tbl, detail = TRUE)
    #> Estimate  CI lower  CI upper      p
    #>    0.017     0.008     0.026  <.001

### Uncertainty coefficient

The uncertainty coefficient (Theil’s U) is based on entropy. It measures how much knowing one variable reduces uncertainty about the other. The p-value tests H0: U = 0 (Wald z-test).

    uncertainty_coef(tbl, detail = TRUE)
    #> Estimate  CI lower  CI upper      p
    #>    0.028     0.016     0.040  <.001

## Yule’s Q

Yule’s Q is defined for 2x2 tables only. It ranges from -1 to +1 and is equivalent to Gamma for 2x2 tables. The p-value tests H0: Q = 0 (Wald z-test).

    tbl_22 <- xtabs(~ smoking + physical_activity, data = sochealth)
    yule_q(tbl_22, detail = TRUE)
    #> Estimate  CI lower  CI upper     p
    #>    0.015    -0.126     0.155  .839

## Automatic selection in cross_tab()

`cross_tab()` can automatically select an appropriate measure via `assoc_measure = "auto"` (the default). When both variables are ordered factors, it picks Kendall’s Tau-b; otherwise it uses Cramer’s V.

    # Nominal: Cramer's V
    cross_tab(sochealth, smoking, education)
    #> Crosstable: smoking x education (N)
    #>
    #>  Values   │   Lower secondary    Upper secondary    Tertiary │   Total
    #> ──────────┼──────────────────────────────────────────────────┼─────────
    #>  No       │               179                415         332 │     926
    #>  Yes      │                78                112          59 │     249
    #> ──────────┼──────────────────────────────────────────────────┼─────────
    #>  Total    │               257                527         391 │    1175
    #>
    #> Chi-2(2) = 21.6, p <.001
    #> Cramer's V = 0.14

    # Ordinal: Kendall's Tau-b (automatic)
    cross_tab(sochealth, self_rated_health, education)
    #> Crosstable: self_rated_health x education (N)
    #>
    #>  Values      │   Lower secondary    Upper secondary    Tertiary │   Total
    #> ─────────────┼──────────────────────────────────────────────────┼─────────
    #>  Poor        │                28                 28           5 │      61
    #>  Fair        │                86                118          62 │     266
    #>  Good        │               102                263         193 │     558
    #>  Very good   │                44                118         133 │     295
    #> ─────────────┼──────────────────────────────────────────────────┼─────────
    #>  Total       │               260                527         393 │    1180
    #>
    #> Chi-2(6) = 73.2, p <.001
    #> Kendall's Tau-b = 0.20

You can override the automatic choice:

    cross_tab(sochealth, self_rated_health, education, assoc_measure = "gamma")
    #> Crosstable: self_rated_health x education (N)
    #>
    #>  Values      │   Lower secondary    Upper secondary    Tertiary │   Total
    #> ─────────────┼──────────────────────────────────────────────────┼─────────
    #>  Poor        │                28                 28           5 │      61
    #>  Fair        │                86                118          62 │     266
    #>  Good        │               102                263         193 │     558
    #>  Very good   │                44                118         133 │     295
    #> ─────────────┼──────────────────────────────────────────────────┼─────────
    #>  Total       │               260                527         393 │    1180
    #>
    #> Chi-2(6) = 73.2, p <.001
    #> Goodman-Kruskal Gamma = 0.31

## Confidence intervals

All functions support confidence intervals via `detail = TRUE`. The confidence level defaults to 95% and can be changed with `conf_level`:

    cramer_v(tbl, detail = TRUE, conf_level = 0.99)
    #> Estimate  CI lower  CI upper      p
    #>    0.176     0.103     0.248  <.001

To get only the estimate and p-value (no CI), pass `conf_level = NULL`:

    cramer_v(tbl, detail = TRUE, conf_level = NULL)
    #> Estimate      p
    #>    0.176  <.001

## Controlling decimal places

When `detail = FALSE` (the default), functions return a plain numeric scalar, so R’s own formatting rules apply. When `detail = TRUE`, the result uses a custom print method that defaults to 3 decimal places. Pass `digits` to change this (the p-value always uses 3 decimal places or `< 0.001`):

    cramer_v(tbl, detail = TRUE, digits = 4)
    #> Estimate  CI lower  CI upper      p
    #>   0.1762    0.1203    0.2309  <.001

The same `digits` argument works for `assoc_measures()`:

    assoc_measures(tbl, digits = 2)
    #> Measure                            Estimate    SE  CI lower  CI upper      p
    #> Cramer's V                             0.18    --      0.12      0.23  <.001
    #> Contingency Coefficient                0.24    --        --        --  <.001
    #> Lambda symmetric                       0.01  0.01      0.00      0.04   .389
    #> Lambda R|C                             0.00  0.00      0.00      0.00     --
    #> Lambda C|R                             0.02  0.03      0.00      0.07   .386
    #> Goodman-Kruskal's Tau R|C              0.02  0.00      0.01      0.03  <.001
    #> Goodman-Kruskal's Tau C|R              0.03  0.01      0.01      0.04  <.001
    #> Uncertainty Coefficient symmetric      0.03  0.01      0.02      0.04  <.001
    #> Uncertainty Coefficient R|C            0.03  0.01      0.01      0.04  <.001
    #> Uncertainty Coefficient C|R            0.03  0.01      0.02      0.04  <.001
    #> Goodman-Kruskal Gamma                  0.31  0.04      0.24      0.38  <.001
    #> Kendall's Tau-b                        0.20  0.03      0.16      0.25  <.001
    #> Kendall's Tau-c                        0.20  0.02      0.15      0.25  <.001
    #> Somers' D R|C                          0.21  0.03      0.16      0.26  <.001
    #> Somers' D C|R                          0.20  0.02      0.15      0.25  <.001

You can also store a result and re-display it with a different precision without recalculating:

    res <- cramer_v(tbl, detail = TRUE)
    print(res, digits = 5)
    #> Estimate  CI lower  CI upper      p
    #>  0.17617   0.12031   0.23092  <.001