def friedman(data=None, dv=None, within=None, subject=None):
"""Friedman test for repeated measurements.
Parameters
----------
data : pandas DataFrame
DataFrame
dv : string
Name of column containing the dependant variable.
within : string
Name of column containing the within-subject factor.
subject : string
Name of column containing the subject identifier.
Returns
-------
stats : DataFrame
Test summary ::
'Q' : The Friedman Q statistic, corrected for ties
'p-unc' : Uncorrected p-value
'dof' : degrees of freedom
Notes
-----
The Friedman test is used for one-way repeated measures ANOVA by ranks.
Data are expected to be in long-format.
Note that if the dataset contains one or more other within subject
factors, an automatic collapsing to the mean is applied on the dependant
variable (same behavior as the ezANOVA R package). As such, results can
differ from those of JASP. If you can, always double-check the results.
Due to the assumption that the test statistic has a chi squared
distribution, the p-value is only reliable for n > 10 and more than 6
repeated measurements.
NaN values are automatically removed.
Examples
--------
Compute the Friedman test for repeated measurements.
>>> from pingouin import friedman, read_dataset
>>> df = read_dataset('rm_anova')
>>> friedman(data=df, dv='DesireToKill', within='Disgustingness',
... subject='Subject')
Source ddof1 Q p-unc
Friedman Disgustingness 1 9.228 0.002384
"""
# Check data
_check_dataframe(dv=dv,
within=within,
data=data,
subject=subject,
effects='within')
# Collapse to the mean
data = data.groupby([subject, within]).mean().reset_index()
# Remove NaN
if data[dv].isnull().any():
data = remove_rm_na(dv=dv,
within=within,
subject=subject,
data=data[[subject, within, dv]])
# Extract number of groups and total sample size
grp = data.groupby(within)[dv]
rm = list(data[within].unique())
k = len(rm)
X = np.array([grp.get_group(r).values for r in rm]).T
n = X.shape[0]
# Rank per subject
ranked = np.zeros(X.shape)
for i in range(n):
ranked[i] = scipy.stats.rankdata(X[i, :])
ssbn = (ranked.sum(axis=0)**2).sum()
# Compute the test statistic
Q = (12 / (n * k * (k + 1))) * ssbn - 3 * n * (k + 1)
# Correct for ties
ties = 0
for i in range(n):
replist, repnum = scipy.stats.find_repeats(X[i])
for t in repnum:
ties += t * (t * t - 1)
c = 1 - ties / float(k * (k * k - 1) * n)
Q /= c
# Approximate the p-value
ddof1 = k - 1
p_unc = scipy.stats.chi2.sf(Q, ddof1)
# Create output dataframe
stats = pd.DataFrame(
{
'Source': within,
'ddof1': ddof1,
'Q': np.round(Q, 3),
'p-unc': p_unc,
},
index=['Friedman'])
col_order = ['Source', 'ddof1', 'Q', 'p-unc']
stats = stats.reindex(columns=col_order)
stats.dropna(how='all', axis=1, inplace=True)
return stats
def cochran(data=None, dv=None, within=None, subject=None):
"""Cochran Q test. Special case of the Friedman test when the dependant
variable is binary.
Parameters
----------
data : pandas DataFrame
DataFrame
dv : string
Name of column containing the binary dependant variable.
within : string
Name of column containing the within-subject factor.
subject : string
Name of column containing the subject identifier.
Returns
-------
stats : DataFrame
Test summary ::
'Q' : The Cochran Q statistic
'p-unc' : Uncorrected p-value
'dof' : degrees of freedom
Notes
-----
The Cochran Q Test is a non-parametric test for ANOVA with repeated
measures where the dependent variable is binary.
Data are expected to be in long-format. NaN are automatically removed
from the data.
The Q statistics is defined as:
.. math:: Q = \\frac{(r-1)(r\\sum_j^rx_j^2-N^2)}{rN-\\sum_i^nx_i^2}
where :math:`N` is the total sum of all observations, :math:`j=1,...,r`
where :math:`r` is the number of repeated measures, :math:`i=1,...,n` where
:math:`n` is the number of observations per condition.
The p-value is then approximated using a chi-square distribution with
:math:`r-1` degrees of freedom:
.. math:: Q \\sim \\chi^2(r-1)
References
----------
.. [1] Cochran, W.G., 1950. The comparison of percentages in matched
samples. Biometrika 37, 256–266.
https://doi.org/10.1093/biomet/37.3-4.256
Examples
--------
Compute the Cochran Q test for repeated measurements.
>>> from pingouin import cochran, read_dataset
>>> df = read_dataset('cochran')
>>> cochran(data=df, dv='Energetic', within='Time', subject='Subject')
Source dof Q p-unc
cochran Time 2 6.706 0.034981
"""
# Check data
_check_dataframe(dv=dv,
within=within,
data=data,
subject=subject,
effects='within')
# Remove NaN
if data[dv].isnull().any():
data = remove_rm_na(dv=dv,
within=within,
subject=subject,
data=data[[subject, within, dv]])
# Groupby and extract size
grp = data.groupby(within)[dv]
grp_s = data.groupby(subject)[dv]
k = data[within].nunique()
dof = k - 1
# n = grp.count().unique()[0]
# Q statistic and p-value
q = (dof * (k * np.sum(grp.sum()**2) - grp.sum().sum()**2)) / \
(k * grp.sum().sum() - np.sum(grp_s.sum()**2))
p_unc = scipy.stats.chi2.sf(q, dof)
# Create output dataframe
stats = pd.DataFrame(
{
'Source': within,
'dof': dof,
'Q': np.round(q, 3),
'p-unc': p_unc,
},
index=['cochran'])
return stats
def friedman(data=None, dv=None, within=None, subject=None, method='chisq'):
"""Friedman test for repeated measurements.
Parameters
----------
data : :py:class:`pandas.DataFrame`
DataFrame
dv : string
Name of column containing the dependent variable.
within : string
Name of column containing the within-subject factor.
subject : string
Name of column containing the subject identifier.
method : string
Statistical test to perform. Must be ``'chisq'`` (chi-square test) or ``'f'`` (F test).
See notes below for explanation.
Returns
-------
stats : :py:class:`pandas.DataFrame`
* ``'W'``: Kendall's coefficient of concordance, corrected for ties
If ``method='chisq'``
* ``'Q'``: The Friedman chi-square statistic, corrected for ties
* ``'dof'``: degrees of freedom
* ``'p-unc'``: Uncorrected p-value of the chi squared test
If ``method='f'``
* ``'F'``: The Friedman F statistic, corrected for ties
* ``'dof1'``: degrees of freedom of the numerator
* ``'dof2'``: degrees of freedom of the denominator
* ``'p-unc'``: Uncorrected p-value of the F test
Notes
-----
The Friedman test is used for one-way repeated measures ANOVA by ranks.
Data are expected to be in long-format.
Note that if the dataset contains one or more other within subject
factors, an automatic collapsing to the mean is applied on the dependent
variable (same behavior as the ezANOVA R package). As such, results can
differ from those of JASP. If you can, always double-check the results.
NaN values are automatically removed.
The Friedman test is equivalent to the test of significance of Kendalls's
coefficient of concordance (Kendall's W). Most commonly a Q statistic,
which has asymptotical chi-squared distribution, is computed and used for
testing. However, in [1]_ they showed the chi-squared test to be overly
conservative for small numbers of samples and repeated measures. Instead
they recommend the F test, which has the correct size and behaves like a
permutation test, but is computationaly much easier.
References
----------
.. [1] Marozzi, M. (2014). Testing for concordance between several
criteria. Journal of Statistical Computation and Simulation,
84(9), 1843–1850. https://doi.org/10.1080/00949655.2013.766189
Examples
--------
Compute the Friedman test for repeated measurements.
>>> from pingouin import friedman, read_dataset
>>> df = read_dataset('rm_anova')
>>> friedman(data=df, dv='DesireToKill', within='Disgustingness',
... subject='Subject')
Source W ddof1 Q p-unc
Friedman Disgustingness 0.099224 1 9.227848 0.002384
This time we will use the F test method.
>>> from pingouin import friedman, read_dataset
>>> df = read_dataset('rm_anova')
>>> friedman(data=df, dv='DesireToKill', within='Disgustingness',
... subject='Subject', method='f')
Source W ddof1 ddof2 F p-unc
Friedman Disgustingness 0.099224 0.978495 90.021505 10.13418 0.002138
We can see, compared to the previous example, that the p-value is slightly
lower. This is expected, since the F test is more powerful (see Notes).
"""
# Check data
_check_dataframe(dv=dv, within=within, data=data, subject=subject,
effects='within')
# Convert Categorical columns to string
# This is important otherwise all the groupby will return different results
# unless we specify .groupby(..., observed = True).
for c in [subject, within]:
if data[c].dtype.name == 'category':
data[c] = data[c].astype(str)
# Collapse to the mean
data = data.groupby([subject, within]).mean().reset_index()
# Remove NaN
if data[dv].isnull().any():
data = remove_rm_na(dv=dv, within=within, subject=subject,
data=data[[subject, within, dv]])
# Extract number of groups and total sample size
grp = data.groupby(within)[dv]
rm = list(data[within].unique())
k = len(rm)
X = np.array([grp.get_group(r).to_numpy() for r in rm]).T
n = X.shape[0]
# Rank per subject
ranked = np.zeros(X.shape)
for i in range(n):
ranked[i] = scipy.stats.rankdata(X[i, :])
ssbn = (ranked.sum(axis=0)**2).sum()
# Correction for ties
ties = 0
for i in range(n):
replist, repnum = scipy.stats.find_repeats(X[i])
for t in repnum:
ties += t * (t * t - 1)
# Compute Kendall's W corrected for ties
W = (12 * ssbn - 3 * n * n * k * (k + 1) * (k + 1)) / (n * n * k * (k - 1) * (k + 1) - n * ties)
if method == 'chisq':
# Compute the Q statistic
Q = n * (k - 1) * W
# Approximate the p-value
ddof1 = k - 1
p_unc = scipy.stats.chi2.sf(Q, ddof1)
# Create output dataframe
stats = pd.DataFrame({'Source': within,
'W': W,
'ddof1': ddof1,
'Q': Q,
'p-unc': p_unc,
}, index=['Friedman'])
elif method == 'f':
# Compute the F statistic
F = W * (n - 1) / (1 - W)
# Approximate the p-value
ddof1 = k - 1 - 2 / n
ddof2 = (n - 1) * ddof1
p_unc = scipy.stats.f.sf(F, ddof1, ddof2)
# Create output dataframe
stats = pd.DataFrame({'Source': within,
'W': W,
'ddof1': ddof1,
'ddof2': ddof2,
'F': F,
'p-unc': p_unc,
}, index=['Friedman'])
return _postprocess_dataframe(stats)