Cross-Sectional Study  -  A study in which separate groups of subjects at different ages are compared. For example, a study in which the performance of a group of three-year-olds on a language comprehension test is compared to the performance of a difference group of four-year-olds.

Experimental Study  -  A study in which one variable is manipulated (the independent variable) and its effect on another variable (the dependent variable) is observed.

Incremental Theory  -  A developmental theory that postulates that development proceeds in a series of steps or a gradual increase in capabilities. For example, information-processing theories.

Longitudinal Study  -  A study in which changes over time in the performance of a single group of subjects is studied. For example, a study in which the same group of children is studied at age three and a year later at age four.

Observational Study  -  A study in which the variables of interest are observed, rather than manipulated. Harder to interpret than an experimental study because other, unmeasured factors, may be the cause of any observed relationship between the variables of interest.

Stage Theory  -  A developmental theory that postulates that development proceeds in a sequence of qualitatively different stages. For example, Piaget's theory of cognitive development.