Proportional rates[edit]
Proportional taxes on consumption are considered by some to be regressive; that is, low income people tend to spend a greater percentage of their income in taxable sales (using a cross section timeframe) than higher income people. A regressive tax is when the average tax rate is lower, with higher income. So income and average tax rate have an inverse relationship. However, this calculation is derived when the tax paid is divided not by the tax base (the amount spent) but by income, which is argued to create an arbitrary relationship. The tax rate itself is proportional, with people with higher incomes paying more tax but at the same rate.
If a consumption tax is to be related to income, the unspent income can be treated as tax-deferred (spending savings at a later point in time), at which time it is taxed creating a proportional rate using an income base. However, consumption taxes such as a sales tax can often exclude items or provide rebates in an effort to create social justice. In many locations, "necessary" items such as non-prepared food, clothing, or prescription drugs are exempt from sales tax to alleviate the burden on the poor.