The leading part in the film Jaws was a man-eating shark. The screening of the film caused a sharp drop in the number of swimmers off the coast of California, where the occasional shark is to be found near the beaches. It has been calculated that the risk of swimmers being snapped up by a shark is very much less than the risk of their being killed in a road accident while on the way to the coast. People do not take account of the true facts – they go upon what makes the deepest impression, or on what first comes to mind.

As a further illustration, consider these two questions: ‘Are there more words with “r” as the first letter than with “r” in the third position?’ ‘Are there more words beginning with “k” than with “k” as the third letter?’ Unless you sense that there is some sort of trick being played, you are likely to answer yes to both questions. But you would be wrong – there are more words with ‘r’ or ‘k’ in the third position than there are words beginning with each letter. The mistake is made because words, both in dictionaries and in our minds, are arranged by their initial letter. It is easy to retrieve from memory words beginning with ‘r’, like ‘roar’, ‘rusty’ and ‘ribald’, but much more difficult to recover words like ‘street’, ‘care’ and ‘borrow’, despite their greater frequency. Lest you think this experiment is unfair because nobody could know the answer without counting up words in a dictionary, here is a variant on it in which knowledge is not involved. The question is, ‘Are there more words ending in “-ing” than ending in “-n-” (that is, having “n” as the penultimate letter)?’ Most people think that ‘-ing’ endings are more common, but in fact ‘-n-’ must be more frequent since all ‘-ing’ endings have ‘n’ as the penultimate letter in addition to many other words (like ‘fine’). People can recall words ending in ‘-ing’ more readily than those ending in ‘-n-’ and they do not stop to go through the simple argument outlined. People also get mixed up with the swapping of words around such as SEO Leeds and then the alternative Leeds SEO.

Judging by the first thing that comes to mind is called the ‘availability error’. I have made it the first error to be described because it permeates all reasoning and, many other specific errors are in reality just further instances of it. Suppose you are thinking of buying a car and you mention it to a friend. He gives you a glowing account of his own car. Deeply impressed, you rush out to buy the same model, only to find that it is totally unreliable and has an outrageous thirst for petrol. The immediacy and salience (availability) of his description have made you forget all the statistics to be found in consumer magazines. You have also committed a second common fallacy, which will be discussed later: no matter how good your friend’s car is, it may or may not be representative of that model in general. No two cars of the same type perform equally well and he may simply have been lucky with his.