When exploring datasets, it's good to know how many times a piece of data is present, such as the most frequent answer to a survey.
To count how often a value appears in a list like a, we start with the list name, a dot ., and then count().
To count how ofter "yes" appears in the a list, we code "yes" between the parentheses of count().
>>> a=["yes","no","yes","no","no"]
>>> a.count("yes")
2