ABSTRACT
Statistical inferential methods are different for qualitative variables than for quantitative variables. This is analogous to different methods of assessment of cardiovascular diseases than for gastrointestinal tract diseases. Those variables can be considered qualitative that are concerned with proportions of subjects in different categories. A variable such as birth weight is, in fact, quantitative but is treated as qualitative when the interest is in knowing the percentage of newborns with weight <2500, 2500–3499, and ≥3500 g. The actual scale can be metric, but the variable becomes qualitative when such categories are formed. There is a rider, though. The number of such categories must be small for it to be qualitative. If, instead of three broad categories, the birth weight is divided into a large number of 100 g categories, such as <2000, 2000–2099, and 2100–2199, the interest would rarely be in a proportion of births in these categories. It would then be in parameters, such as mean or median birth weight. The methods to draw inferences for such quantitative parameters are presented in Chapter 15.
