Summing nominal or ordinal indicators is statistically invalid without an interval-scale argument.

Evidence

It is extremely common for researchers to sum nominal or ordinal indicators to form an index of a concept: this is not strictly speaking valid. One needs to make the argument that these nominal or ordinal variables are in fact interval. Similarly, scholars like to multiply or divide variables to form indices: this is not valid unless the variables are ratio. — Gary Goertz, Social Science Concepts and Measurement, 908

Categorization

• SHARE: Send to the methods-quant colleague who routinely sums Likert items into composite scales without justifying intervalness.