Income mobility is usually considered a good thing. It implies higher social welfare as the ability of individuals to move up and down the income ladder mitigates the impacts of poor income distribution. But it is also true that when income jumps up and down unexpectedly, life becomes riskier and planning, difficult. This is why making a general link between the mobility we observe in the data and welfare is not straightforward.
A common approach used to show high mobility is a low correlation of present and past incomes is captured, for instance, by the Hart index (cov lnyt, lnyt-1). If we assume, as is often done, that an individual’s income is comprised of a transitory component (short-term blips up or down in a self-employed person’s income that we can smooth, or even measurement error), and a permanent component where each income shock is persistent (say, an income loss after an involuntary job change (an AR (1) process with autoregressive coefficient, ρ), then the Hart index can be broken into three parts.