I've come across numerous tables published online categorizing Cramér's V effect sizes for categorical tables with degrees of freedom $\leq$ 5, for example:

$$

\begin{array}{c|lcr}

\text{Degrees of freedom} & \text{Negligible} & \text{Small} & \text{Medium} & \text{Large}\\

\hline

\text{1} & \lt 0.10 & [0.10, 0.30) & [0.30, 0.50) & 0.50+\\

\text{2} & \lt 0.07 & [0.07, 0.21) & [0.21, 0.35) & 0.35+\\

\text{3} & \lt 0.06 & [0.06, 0.17) & [0.17, 0.29) & 0.29+\\

\text{4} & \lt 0.05 & [0.05, 0.15) & [0.15, 0.25) & 0.25+\\

\text{5} & \lt 0.05 & [0.05, 0.13) & [0.13, 0.22) & 0.22+\\

\end{array}

$$

What are the "Negligible/Small/Medium/Large" rules of thumb for interpreting Cramér's V effect sizes for degrees of freedom > 5?

## Best Answer

The table you reproduced relies on Cohen, J. (1988).

Statistical power analysis for the behavioral sciences(2nd ed).If you want to follow Cohen's rule of thumb (which may or may be not a good idea) for any given degrees of freedom, first you have to convert Cramér's V to Cohen's omega ($\omega$).

You do that by multiplying $V$ by the square root of the table's degrees of freedom. Cohen gives this formula in his book (formula 7.2.7, page 223):

where $\phi '$ is Cramér's V, and $(r-1)$ is the degrees of freedom ($r$ being the contingency table's smallest dimension, not necessarily the number of rows, contrary to what the letter "r" might lead one to believe).

Then, you can use the rule of thumb he describes pp. 224-227 for interpreting $\omega$, which is:

As an example: if you have a $9 \times 8$ table with $V = 0.2$, you get $\omega = 0.2 \times \sqrt{8-1} \approx 0.53$, which is a large effect size according to Cohen.

The table you mention in your question is essentialy an aid to avoid having to calculate $\omega$ yourself (which isn't really complicated if you have a computer at your disposal). It seems that the table is constructed from the table 7.2.3 on page 222 of Cohen's book, where Cohen shows various equivalents of $\omega$ in terms of Cramer's V, up to 5 degrees of freedom (I guess that saving ink and space was the main rationale for limiting the examples to 5 degrees of freedom).

You can easily check that the table you copied relies on Cohen's formula, for example if you take the line "4 degrees of freedom": $0.15 \times \sqrt{4} = 0.3$ (which is a medium effect for $\omega$), or if you take the line "5 degrees of freedom" with $0.22 \times \sqrt{5} \approx 0.5$ (large effect for $\omega$).

Note that contrary to Cramér's V, Cohen's $\omega$ does not have a upper bound, so it's entirely possible to end up with results where $\omega > 1$.

Also note that Cohen gives a word of caution about using his rule of thumb, page 224 of his book:

and: