An extended study of the nonindependence of stimulus properties in human classification learning

Categorization researchers have tried to verify their models through laboratory experiments with simplified stimulus sets, a requirement that can rarely be met in real-world situations in which properties are often connected. Still, the targeted simplification of the material might be illusory. We replicate and extend Love and Markman's (2003) study of the nonindependence of canonical stimulus properties such as size, colour, and shape in human classification learning, in which the authors concluded that shape takes precedence over other dimensions. To support their hypothesis, Love and Markman showed that certain classifications are more difficult for participants when shape is combined to one of its putative subordinate features, size or colour, than when shape is irrelevant to the task. A data set of 290 + 50 adult participants completing one or more classification tasks was collected. The results confirm that certain combinations of shape, size, and colour can hinder or facilitate classification learning, but not necessarily in the form expected by the nonindependence postulated by Love and Markman, especially in Experiment 2 where a totally reverse pattern of difficulty is observed (shape does not take precedence over other dimensions). Also, we show that simple similarity effects in clustering retain considerable intuitive appeal and can offer an alternative account to the nonindependence of stimulus properties, especially because slight variations in the dimensions chosen make the observations of Love and Markman unstable.