Sentence:  Although there is an abundance of statistical literature describing (complicated) methods that can be used to investigate whether or not one is dealing with <WSD>ignorable</WSD> or informative missing data in a longitudinal study (see, for instance, Diggle, 1989; Ridout, 1991; Diggle et al., 1994), it is basically quote easy to investigat this matter. 
Meaning of the word marked with <WSD> tags:  Unrelated to the parameters being estimated