- To answer any psychological question, first people need to be measured
- Quantify psychological phenomena
- Commonly done through psychological tests or questionnaires
- Persons answer questions which can be rated or scored
- Can be administered in several ways:
- Administer the test to a lot of persons once
- Administer the test to the same person multiple times
- Repeated measures / longitudinal data
- Administer the test to multiple people multiple times
- Consistently in most Psychological tests and questionnaires, item scores correlate positively with each-other
- A child who answers one summation item correctly is likely to answer another correctly
- Someone who "does a thorough job" tends not to "be lazy"
- These positive correlations are classically modeled by having the items be the causal result of some latent factor
- The correlation between any pair of items is modeled as being solely due to the common influence of the latent trait
- The field of psychometrics is, mostly, devoted to measuring such latent traits such that they can be used in psychological experiments, growth modeling, scoring/examination, etc.