As a very extreme case of self-selection, imagine a five-foot-high solid wood fence surrounding a collection of Great Danes and Miniature Poodles. You want to estimate the proportion of Great Danes inside and decide to collect your sample by observing the first seven dogs to jump high enough to be seen above the fence.
(a) Explain how this is a self-selected sample that is, of course, very misleading.
(b) How is this sample selection procedure like a call-in election poll?