statsmodels.stats.rates.test_poisson_2indep¶

statsmodels.stats.rates.test_poisson_2indep(count1, exposure1, count2, exposure2, value=None, ratio_null=None, method=None, compare='ratio', alternative='two-sided', etest_kwds=None)[source]¶

Test for comparing two sample Poisson intensity rates.

Rates are defined as expected count divided by exposure.

The Null and alternative hypothesis for the rates, rate1 and rate2, of two independent Poisson samples are

for compare = ‘diff’

H0: rate1 - rate2 - value = 0
H1: rate1 - rate2 - value != 0 if alternative = ‘two-sided’
H1: rate1 - rate2 - value > 0 if alternative = ‘larger’
H1: rate1 - rate2 - value < 0 if alternative = ‘smaller’

for compare = ‘ratio’

H0: rate1 / rate2 - value = 0
H1: rate1 / rate2 - value != 0 if alternative = ‘two-sided’
H1: rate1 / rate2 - value > 0 if alternative = ‘larger’
H1: rate1 / rate2 - value < 0 if alternative = ‘smaller’

Parameters:¶

count1 : int¶

Number of events in first sample, treatment group.

exposure1 : float¶

Total exposure (time * subjects) in first sample.

count2 : int¶

Number of events in second sample, control group.

exposure2 : float¶

Total exposure (time * subjects) in second sample.

ratio_null : float¶

Ratio of the two Poisson rates under the Null hypothesis. Default is 1. Deprecated, use value instead.

Deprecated since version 0.14.0: Use value instead.

value : float¶

Value of the ratio or difference of 2 independent rates under the null hypothesis. Default is equal rates, i.e. 1 for ratio and 0 for diff.

Added in version 0.14.0: Replacement for ratio_null.

method : string¶

Method for the test statistic and the p-value. Defaults to ‘score’. see Notes.

ratio:

’wald’: method W1A, wald test, variance based on observed rates
’score’: method W2A, score test, variance based on estimate under the Null hypothesis
’wald-log’: W3A, uses log-ratio, variance based on observed rates
’score-log’ W4A, uses log-ratio, variance based on estimate under the Null hypothesis
’sqrt’: W5A, based on variance stabilizing square root transformation
’exact-cond’: exact conditional test based on binomial distribution
This uses binom_test which is minlike in the two-sided case.
’cond-midp’: midpoint-pvalue of exact conditional test
’etest’ or ‘etest-score: etest with score test statistic
’etest-wald’: etest with wald test statistic

diff:

’wald’,
’waldccv’
’score’
’etest-score’ or ‘etest: etest with score test statistic
’etest-wald’: etest with wald test statistic

compare : {'diff', 'ratio'}¶

Default is “ratio”. If compare is ratio, then the hypothesis test is for the rate ratio defined by ratio = rate1 / rate2. If compare is diff, then the hypothesis test is for diff = rate1 - rate2.

alternative : {"two-sided" (default), "larger", smaller}¶

The alternative hypothesis, H1, has to be one of the following

’two-sided’: H1: ratio, or diff, of rates is not equal to value
’larger’ : H1: ratio, or diff, of rates is larger than value
’smaller’ : H1: ratio, or diff, of rates is smaller than value

etest_kwds : dictionary¶

Additional optional parameters to be passed to the etest_poisson_2indep function, namely y_grid.

Returns:¶

results – The two main attributes are test statistic statistic and p-value pvalue.

Return type:¶

instance of HolderTuple class