if the major causes of variation in incomes are transitory in nature,

The total sample is divided into seven income groups (Gl-G7). The assignment to groups is one a one-time basis and according to the simple arithmetic mean income of the individual household over the 13-ycar period. These real income levels begin with mean incomes of less than $4999, and increasing in increments of $5000. The last group contains mean incomes of $35,000 or more.