monthglm function

Fit a GLM with Month

Fit a generalized linear model with a categorical variable of month.

monthglm(
  formula,
  data,
  family = gaussian(),
  refmonth = 1,
  monthvar = "month",
  offsetmonth = FALSE,
  offsetpop = NULL
)

Arguments

• formula: regression model formula, e.g., y~x1+x2, (do not add month to the regression equation, it will be added automatically).

• data: a data frame.

• family: a description of the error distribution and link function to be used in the model (default=gaussian()). (See family

  for details of family functions.).

• refmonth: reference month, must be between 1 and 12 (default=1 for January).

• monthvar: name of the month variable which is either an integer (1 to 12) or a character or factor (Jan' to Dec' or January' to December') (default='month').

• offsetmonth: include an offset to account for the uneven number of days in the month (TRUE/FALSE). Should be used for monthly counts (with family=poisson()).

• offsetpop: include an offset for the population (optional), this should be a variable in the data frame. Do not log-transform the offset as the log-transform is applied by the function.

Returns

• call: the original call to the monthglm function.

• fit: GLM model. - fitted: fitted values.

• residuals: residuals. - out: details on the monthly estimates.

Details

Month is fitted as a categorical variable as part of a generalized linear model. Other independent variables can be added to the right-hand side of formula.

This model is useful for examining non-sinusoidal seasonal patterns. For sinusoidal seasonal patterns see cosinor.

The data frame should contain the integer months and the year as a 4 digit number. These are used to calculate the number of days in each month accounting for leap years.

Examples

data(CVD)
mmodel = monthglm(formula=cvd~1 ,data=CVD, family=poisson(),
                  offsetpop=expression(pop/100000), offsetmonth=TRUE)
summary(mmodel)

References

Barnett, A.G., Dobson, A.J. (2010) Analysing Seasonal Health Data. Springer.

See Also

summary.monthglm, plot.monthglm

Author(s)

Adrian Barnett a.barnett@qut.edu.au