Judging frequency and probability  In general, availability is correlated with ecological frequency, but it is
also affected by other factors. Consequently, the reliance on the availability
heuristic leads to systematic biases. Such biases are demonstrated in the
judged frequency of classes of words, of combinatoric outcomes, and of
repeated events. The phenomenon of  illusory correlation
 is explained as an
availability bias.  [8]

In the original Tversky and Kahneman (1973) research, three major factors that
are discussed are the frequency of repetition, frequency of co-occurrence, and
illusory correlation. The use of frequency of repetition aids in the retrieval
of relevant instances. The idea behind this phenomenon is that the more an
instance is repeated within a category or list, the stronger the link between
the two instances becomes. Individuals then use the strong association between
the instances to determine the frequency of an instance. Consequently, the
association between the category or list and the specific instance often
influences frequency judgement. Frequency of co-occurrence strongly relates to
Frequency of repetition, such that the more an item-pair is repeated, the
stronger the association between the two items becomes, leading to a bias when
estimating the frequency of co-occurrence. Due to the phenomena of frequency
of co-occurrence, illusory correlations also often play a big role.  [8]

Another factor that affects the availability heuristic in frequency and
probability is exemplars. Exemplars are the typical examples that stand out
during the process of recall. If asked what participants thought different set
sizes were (how many men and how many women are in the class), participants
would use exemplars to determine the size of each set. Participants would
derive their answers on ease of recall of the names that stood out.
Participants read a list of names of members of a class for 30 seconds, and
then participants were asked the male to female ratio of the class. The
participant's answer would depend on the recall of exemplars. If the
participant reading the list recalled seeing more common male names, such as
Jack, but the only female names in the class were uncommon names, such as
Deepika, then the participant will recall that there were more men than women.
The opposite would be true if there were more common female names on the list
and uncommon male names. Due to the availability heuristic, names that are
more easily available are more likely to be recalled, and can thus alter
judgments of probability.  [31]

Another example of the availability heuristic and exemplars would be seeing a
shark in the ocean. Seeing a shark has a greater impact on an individual's
memory than seeing a dolphin. If someone sees both sharks and dolphins in the
ocean, they will be less aware of seeing the dolphins, because the dolphins
had less of an impact on their memory. Due to the greater impact of seeing a
shark, the availability heuristic can influence the probability judgement of
the ratio of sharks and dolphins in the water. Thus, an individual who saw
both a shark and a dolphin would assume a higher ratio of sharks in the water,
even if there are more dolphins in reality.  [31]