What is a key measure of a learning aid's validity?

A key measure of a learning aid's validity is the extent to which it meets the manufacturers' claims. This means that the learning aid should effectively provide the benefits and outcomes that it promises to its users. When a product claims to improve a specific skill or understanding, validity is assessed based on how accurately it delivers on that promise. This ensures that the aid is not just effective in theory but also useful and relevant in practice for the intended learning objectives.

The other options, while they may contribute to the overall quality and usability of the learning aid, do not directly measure its validity. The complexity of the content it covers may determine the level of difficulty but does not speak to whether it fulfills its claims. The duration of time required to complete the aid can be important for practical use, but it does not measure how valid or effective the aid is in achieving learning goals. Similarly, the age group for which the aid is designed helps determine appropriateness, but it does not directly address the alignment of the aid with its advertised outcomes.