Re’cen•cy Bi•as n. is when a person assigns greater value to the most recent piece of information observed.

An example from Tufte follows:

…when this board is mentioned at a major site (slashdot, Arts & Letters Daily, Kottke, NYTimes), our logs will show, say, 2000 visits from that site the first day, a 1000 the second, and then, as the mention slips into a screen position reachable only by scrolling, 100 on the third day, and by 5 days, nothing. The quality of the thread didn’t change in 5 days, only its relative position on the referring site.

Recency bias is also known as recency effect, which is in direct opposition to primacy effect (placing a greater value on the first piece(s) of information a person observes). Another way to explain it is this example from Wikipedia:

…if a driver sees an equal total number of red cars as blue cars during a long journey, but there happens to be a glut of red cars at the end of the journey, he or she is likely to conclude that there were more red cars than blue cars throughout the drive.

The two effects in conjunction, referred to as serial position effect, are illustrated in the informational graph below:

This graph from Wikipedia shows how primacy and recency effect word recall.

When some sort of unrelated task is assigned after an experience or observation, like solving a math problem in your head, it can reduce the effect of recency bias. I assume this is because information that would encourage a recency bias is stored in the same area of the brain utilized to solve the math problem.

Connected, it seems, is the law of small numbers. Defined by Wikipedia, the law of small numbers is the tendency to extrapolate big conclusions from small samples. An example, from the same wikipedia page, is:

…sports fans assuming that a few excellent performances are proof of a player’s underlying ability… or … would be investors, who may assume that a mutual fund’s record over one year is a reliable indicator of the manager’s skill.

Also relevant are the assumptions of the Belief-Adjustment Model by Hogarth and Einhorn referenced in the article Effects of information sequence and irrelevant distractor information when using a computer-based decision aid by Reneau and Blanthorne:

1. The sequence in which information is presented affects judgments in a predictable manner.
2. The presence of irrelevant distractor information reduces judgment accuracy.
3. Neither information sequence nor irrelevant distractor information affect judgment confidence.

The point is this: Recency bias is a tool we can use to improve user comprehension and overall satisfaction. By considering the sequence of information (keeping in mind that many times the “home page” of your website is not the landing page of users.) and the (intentional?) placement of distractors, we can avoid common pitfalls in user judgments and decision making. And holy cow! Recency bias is but one of an entire list of cognitive biases.