there have been criticisms, raising the point that association does not prove causality, but the few studies that have included a longitudinal element have also tended to support the view that the practices impact on performance rather than the other way round eg in the UK, Patterson et al. (1995) using the WERS data set found that implementation of HPWPs was followed by improvements in performance, a finding echoed in the US by Ichniowski et al. (2004). these longitudinal studies also suggest that there is a lagged effect with the implementation of the practices taking some time to filter through to performance improvement.