Sargan Eight is a statistical test used in econometrics to assess the validity of instrumental variables in regression models. It checks whether the instruments used are uncorrelated with the error term, which is crucial for ensuring that the estimates are unbiased and consistent. The test is named after the economist David Sargan, who developed it in the 1950s. The Sargan test is particularly useful in models where endogeneity is a concern, such as in panel data or time series analysis. A significant result from the test suggests that the instruments may not be valid, prompting researchers to reconsider their choice of instruments to improve model reliability.