Jakob Nielsen spots another-shaped Tail, this time the lesser-spotted Slow Tail, referring to the conversion time for buyers to complete a transaction after their initial visit to a website:
"A full 5% of orders occur more than four weeks after users click on search engine ads.
...Although 75% of the conversions occurred within 24 hours, the last quarter took much longer to arrive. Orders didn't reach the 90% mark until 12 days after users had clicked on the advertisement, and it took four weeks to reach 95%. Thus, the last 5% of orders happened more than four weeks after the initial click.
The following chart shows the days required to reach a certain percentage of the ultimate conversions."
Via Lawrence Lee.