country invariance of compensating differentials as a share of income is also supported by research showing that elasticities are relatively similar across countries with a similar level of economic development helliwell et al a fixedeffects framework also reduces the risk of biased estimates dueunobserved heterogeneity ie omitted timeinvariant effects that are correlated to the regressors introducing fixedeffects is indeed equivalent to regressing the change in life satisfaction on the change in explanatory variables in the sample of countries under study there appears to be very low correlation ie below between changes in log income longevity and unemployment and hence no risk of encountering multicollinearity problems