Parameter Description: Type of Design
The type of design. Type of design is one of the following:
O'Brien & Fleming ("OF"), Pocock ("P"), Wang & Tsiatis Delta class ("WT"),
Haybittle & Peto ("HP"), Optimum design within Wang & Tsiatis class ("WToptimum"),
O'Brien & Fleming type alpha spending ("asOF"), Pocock type alpha spending ("asP"),
Kim & DeMets alpha spending ("asKD"), Hwang, Shi & DeCani alpha spending ("asHSD"),
user defined alpha spending ("asUser"), default is "OF".