The following formulas are used:

Percent Change
((x(t)/x(t-1)) - 1) * 100

Percent Change from Year Ago
((x(t)/x(t-n_obs_per_yr)) - 1) * 100

Change from Year Ago (where the indicator is already in percent. ie: Unemployment Rate)
x(t) - x(t-n_obs_per_yr)


'x(t)' is the value of series x at time period t.

'n_obs_per_yr' is the number of observations per year. The number of observations per year differs by frequency:

Annual, 1
Monthly, 12
Quarterly, 4