One way to combat adverse selection, firms will try to use signalling, pioneered by Michael Spence, whereby employers could use various characteristics of applicants differentiate between high-ability or low-ability workers. One common signal used is education, whereby employers assume that high-ability workers will have higher levels of education. Employers can then compensate high-ability workers with higher wages. However, signalling does not always work, and it may appear to an external observer that education has raised the marginal product of labour, without this necessarily being true.