This Working Paper addresses a key issue for research on disability issues and policies: the volatility of the measurement of disability and its impact on indicators based on this measure, such as employment rates and employment gaps. People in different countries and in the same country at different times differ in how likely they are to report a disability. This makes it difficult if not impossible to measure the impact of policies on disability employment outcomes. Even worse, a successful policy may change the way that people report disability, in ways that make the disability employment gap look worse even though the policy has been a success. To overcome these issues, this report discusses different ways of producing more reliable estimates of disability employment rates and gaps and trends over time, to complement the disability employment gap indicator.