At the most abstract: anything on which things may have different values. Typically in our field these are things on which people may have different values: height, weight, age, pre-therapy questionnaire scores are all examples of “continuous” variables in that there are many possible scores (if the questionnaire is any good!) By contrast “attended” is a binary variable describing a therapy session: someone either did attend or not (whereas lateness is a continuous variable).
Generally in our field a variable may have missing values, e.g. where someone did not complete a measure. Continuous variables are often separated into ratio, interval and ordinal though the distinction is often overvalued. Binary variables are a particular set among categorical variables and categorical variables, for example, modality of therapy, may have a number of values (greater than two otherwise they are binary categorical variables) and there is no clear ordering of the values.
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