Developed to perform the estimation and inference for regression
coefficient parameters in longitudinal marginal models using the method of
quadratic inference functions. Like generalized estimating equations, this
method is also a quasi-likelihood inference method. It has been showed that
the method gives consistent estimators of the regression coefficients even if
the correlation structure is misspecified, and it is more efficient than GEE
when the correlation structure is misspecified. Based on Qu, A., Lindsay,
B.G. and Li, B. (2000)