- scipy.stats.contingency.chi2_contingency(observed, correction=True, lambda_=None)[source]#
Chi-square test of independence of variables in a contingency table.
This function computes the chi-square statistic and p-value for the hypothesis test of independence of the observed frequencies in the contingency table  observed. The expected frequencies are computed based on the marginal sums under the assumption of independence; see
scipy.stats.contingency.expected_freq. The number of degrees of freedom is (expressed using numpy functions and attributes):
dof = observed.size - sum(observed.shape) + observed.ndim - 1
The contingency table. The table contains the observed frequencies (i.e. number of occurrences) in each category. In the two-dimensional case, the table is often described as an “R x C table”.
- correctionbool, optional
If True, and the degrees of freedom is 1, apply Yates’ correction for continuity. The effect of the correction is to adjust each observed value by 0.5 towards the corresponding expected value.
- lambda_float or str, optional
By default, the statistic computed in this test is Pearson’s chi-squared statistic . lambda_ allows a statistic from the Cressie-Read power divergence family  to be used instead. See
An object containing attributes:
The test statistic.
The p-value of the test.
The degrees of freedom.
- expected_freqndarray, same shape as observed
The expected frequencies, based on the marginal sums of the table.
An often quoted guideline for the validity of this calculation is that the test should be used only if the observed and expected frequencies in each cell are at least 5.
This is a test for the independence of different categories of a population. The test is only meaningful when the dimension of observed is two or more. Applying the test to a one-dimensional table will always result in expected equal to observed and a chi-square statistic equal to 0.
This function does not handle masked arrays, because the calculation does not make sense with missing values.
scipy.stats.chisquare, this function computes a chi-square statistic; the convenience this function provides is to figure out the expected frequencies and degrees of freedom from the given contingency table. If these were already known, and if the Yates’ correction was not required, one could use
scipy.stats.chisquare. That is, if one calls:
res = chi2_contingency(obs, correction=False)
then the following is true:
(res.statistic, res.pvalue) == stats.chisquare(obs.ravel(), f_exp=ex.ravel(), ddof=obs.size - 1 - dof)
The lambda_ argument was added in version 0.13.0 of scipy.
“Contingency table”, https://en.wikipedia.org/wiki/Contingency_table
“Pearson’s chi-squared test”, https://en.wikipedia.org/wiki/Pearson%27s_chi-squared_test
Cressie, N. and Read, T. R. C., “Multinomial Goodness-of-Fit Tests”, J. Royal Stat. Soc. Series B, Vol. 46, No. 3 (1984), pp. 440-464.
A two-way example (2 x 3):
>>> import numpy as np >>> from scipy.stats import chi2_contingency >>> obs = np.array([[10, 10, 20], [20, 20, 20]]) >>> res = chi2_contingency(obs) >>> res.statistic 2.7777777777777777 >>> res.pvalue 0.24935220877729619 >>> res.dof 2 >>> res.expected_freq array([[ 12., 12., 16.], [ 18., 18., 24.]])
Perform the test using the log-likelihood ratio (i.e. the “G-test”) instead of Pearson’s chi-squared statistic.
>>> res = chi2_contingency(obs, lambda_="log-likelihood") >>> res.statistic 2.7688587616781319 >>> res.pvalue 0.25046668010954165
A four-way example (2 x 2 x 2 x 2):
>>> obs = np.array( ... [[[[12, 17], ... [11, 16]], ... [[11, 12], ... [15, 16]]], ... [[[23, 15], ... [30, 22]], ... [[14, 17], ... [15, 16]]]]) >>> res = chi2_contingency(obs) >>> res.statistic 8.7584514426741897 >>> res.pvalue 0.64417725029295503