zepid.calc.utils.incidence_rate_difference(a, c, t1, t2, alpha=0.05)

Calculates the incidence rate difference and confidence intervals from count data

Incidence rate difference is calculated from

\[ID = \frac{a}{t_1} - \frac{c}{t_2}\]

Incidence rate difference standard error is

\[SE = \left(\frac{a}{t_1^2} + \frac{c}{t_2^2}\right)^{\frac{1}{2}}\]
  • a (integer, float) – Count of exposed individuals with outcome
  • c (integer, float) – Count of unexposed individuals with outcome
  • t1 (integer, float) – Person-time contributed by those who were exposed
  • t2 (integer, float) – Person-time contributed by those who were unexposed
  • alpha (float, optional) – Alpha value to calculate two-sided Wald confidence intervals. Default is 95% confidence interval

Tuple of ID, lower CL, upper CL, SE)

Return type:



Estimate the incidence rate ratio, standard error, and confidence intervals

>>> from zepid.calc import incidence_rate_difference
>>> ird = incidence_rate_difference(45, 21, 109, 158)

Extracting the estimated incidence rate ratio

>>> ird.point_estimate

Extracting the lower and upper confidence intervals, respectively

>>> ird.lower_bound
>>> ird.upper_bound

Extracting the standard error

>>> ird.standard_error