In any study, there is a theoretical relationship among constructs. For instance, the correlation motivation and job performance. However, in practice, or in an empirical study, we need to use a measure as an indicator for motivation and one for the job performance.
Construct validity is concerned about how well the indicators represent the theoretical constructs.
Since we can only see the relationship I'-D', we use these results to make inferences. If I' does not represent I well or if D' does not represent D well then the inference made from I'-D' will be incorrect or inaccurate. Thus, we must make sure that I' and D' are good proxies of I and D respectively. Thus, we must reduce contamination and deficiency when developing measures. Construct validity is the extent to which the measures correspond with the actual construct.