Which of the following best describes an interval variable?

An interval variable is characterized by having equally spaced intervals between values, which allows for meaningful comparisons of differences between the values. This means that not only can you rank the values of an interval variable, but you can also quantify the actual differences between them. For example, in the case of temperature measured in Celsius, the difference between 20°C and 30°C is the same as the difference between 30°C and 40°C, demonstrating equal intervals.

Other options describe different types of variables. Categorical groups refer to nominal variables, which do not have a numerical value. The mention of clear ordering but no consistent spacing applies to ordinal variables, where rank order is evident, but the intervals between ranks are not consistent or defined. Lastly, ordered categories without defined intervals also refer to ordinal variables. These distinctions are crucial for understanding how to choose the proper statistical methods for analysis based on the type of variable in use.