<qa>  What is the Lan-DeMets procedure flexible?The Lan-DeMets procedure is flexible in that the total number and timing of the sequential analyses need not be specified in advance. Also, there is no requirement that looks be equally spaced. In some clinical trials, however, a surprising development may lead to extra looks or even continuous monitoring of data. The Lan-DeMets procedure is flexible, in that it can switch from occasional to continuous monitoring of the data with negligible effect on the Type I error level. 