I've heard from several sources that the predominant form of alternative splicing (at least in mammals) is exon skipping. However, my personal evidence is only anecdotal: I've heard it and read it, but have not seen references to literature. Is there a seminal paper or set of papers establishing this finding that I can cite?
You can see this review.
There are several different types of alternative splicing (AS) events, which can be classified into four main subgroups. The first type is exon skipping, in which a type of exon known as a cassette exon is spliced out of the transcript together with its flanking introns (see the figure, part a). Exon skipping accounts for nearly 40% of AS events in higher eukaryotes [17, 111] but is extremely rare in lower eukaryotes.