Table 1:

Hypothetical data with three age categories and three periods illustrating a shift in meaning and interpretation of interaction terms under different coding schemes. The interaction terms under different types of coding in cell Ƴ necessarily have different numerical values because of different reference groups. For example, the interaction term in cell Ƴ under dummy coding represents a directional difference from a particular reference group (e.g., age 20–24 in year 2000). Under effect coding, the same interaction term in cell Ƴ represents the deviation in the outcome from the age main effect plus period main effect for the group of individuals who were age 25–29 and surveyed in period 2005. Such different numeric values do not arise from an identification problem but rather from a shift in what these quantities represent.

		Period
		2000	2005	2010
Age	20–24
	25–29		Ƴ
	30–34