The Herald interactive for election results looks really impressive. The headline infographic for the latest poll, not so much. The graph is designed to display changes between two polls, for which the margin of error is 1.4 times higher than in a single poll: the margin of error for National goes beyond the edge of the graph.
The lead for the story is worse
The Kim Dotcom-inspired event in Auckland’s Town Hall that was supposed to end John Key’s career gave the National Party an immediate bounce in support this week, according to polling for the last Herald DigiPoll survey.
Since both the Dotcom and Greenwald/Snowden Moments of Truth happened in the middle of polling, they’ve split the results into before/after Tuesday. That is, rather than showing an average of polls, or even a single poll, or even a change from a single poll, they are headlining the change between the first and second halves of a single poll!
The observed “bounce” was 1.3%. The quoted margin of error at the bottom of the story is 3.5%, from a poll of 775 people. The actual margin of error for a change between the first and second halves of the poll is about 7%.
Only in the Internet Party’s wildest dreams could this split-half comparison have told us anything reliable. It would need the statistical equivalent of the CSI magic video-zoom enhance button to work.