The index is updated after each election cycle. Cook provided the following explanation of a sample PVI in its 2017 PVI summary: !function(){"use strict";window.addEventListener("message",(function(e){if(void 0!==e.data["datawrapper-height"]){var t=document.querySelectorAll("iframe");for(var a in e.data["datawrapper-height"])for(var r=0;r