Which statement is incorrect regarding the 1988 World Bank study on household demand for outpatient services in Ogun State?

The question asks to identify the incorrect statement regarding the 1988 World Bank study on household demand for outpatient services in Ogun State. We need to find the statement that is not true.

Let's analyze each option:

• a) "Some of the factors implicated, are: price of the care, quality of the care, sex and education of the patients, wealth of the household, income of the household." These are all standard socioeconomic and service-related factors commonly considered in studies on healthcare demand. This statement is likely true.
• b) "Other factors studied were urban residence, symptoms and seriousness of the illness." These are also common factors influencing healthcare demand, relating to access (urban residence) and perceived need (symptoms, seriousness of illness). This statement is likely true.
• c) "The empirical model assumed that choice of health care is a function of many factors." This is a fundamental principle in econometric modeling of healthcare demand, where multiple variables are used to explain health-seeking behavior. This statement is highly likely to be true for such an extensive study.
• d) "This was a biased study." Without specific information about the methodology, data collection, or analysis of the 1988 World Bank study, it is impossible to definitively conclude that it was biased. Reputable institutions like the World Bank typically aim for rigorous and unbiased research. Stating that a study was biased without any evidence or context is a strong claim and is often the incorrect statement in such questions, especially when other options describe plausible aspects of a research study.

Given that options a, b, and c describe typical and reasonable characteristics of a study on healthcare demand, the unsubstantiated claim in option d is the most likely exception or incorrect statement.

The final answer is $\boxed{\text{d}}$