class inclusion

the concept that a subordinate class (e.g., dogs) must always be smaller than the superordinate class in which it is contained (e.g., animals). Jean Piaget believed that understanding the concept of class inclusion represented an important developmental step. Children progress from classifications based on personal factors, perceptual features, and common function to classifications based on hierarchical relationships; for example, a monkey is a primate, a mammal, and a vertebrate animal. See concrete operation.

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Psychology term of the day

March 11th 2025

quasi-experimental design

an experimental design in which assignment of participants to an experimental group or to a control group cannot be made at random for either practical or ethical reasons; this is usually the case in field research. Assignment of participants to conditions is usually based on self-selection (e.g., employees who have chosen to work at a particular plant) or selection by an administrator (e.g., children are assigned to particular classrooms by a superintendent of schools). Such designs introduce a set of assumptions or threats to internal validity that must be acknowledged by the researcher when interpreting study findings. A study using this design is called a quasi-experiment. Examples include studies that investigate the responses of large groups to natural disasters or widespread changes in social policy.