Four preregistered studies test whether GPT-4o-mini-2024-07-18 can generate synthetic participant responses useful for scale development before human data collection. The model receives demographic profiles based on representative UK quotas and answers three reworded prompts per item at temperature 1; item responses are averaged. Each study compares about 300 real and 300 simulated participants: 316/322 for a climate scale, 331/331 for ICT-SC25, 301/300 for SAGAT, and 301/300 for AI Anxiety. Study 1 fails to reproduce the factor structure or invariance. Study 2 reproduces the structure with configural and partial metric invariance, but matching-dimension correlations are only 0.21–0.35 and individual-level ICC is 0.19. In Studies 3 and 4, EFA on synthetic data proposes structures that obtain good CFA fit in newly collected human samples and reach approximate configural, metric, and scalar invariance, but not residual invariance. Across all studies, distributional and variance discrepancies remain, while human–synthetic correlations are weak or near zero. The evidence therefore supports, at most, exploratory group-level early prototyping of factor structures; it does not support replacing human validation or simulating individual responses. Internal gender invariance within synthetic data does not establish human fairness and may simply reflect generator regularity. The work uses one model, one country, English, and Likert scales; invalid outputs are imputed by averaging other prompts, and Study 1 includes an acknowledged poorly specified hypothesis. The OSF links were checked, but their public API did not expose an immutable, auditable code-and-data package.
Research question
Can a synthetic population generated by an LLM reproduce factorial structures and human measurement properties with utility for the initial development of scales?