The presence of common cost raises some problems for cost-of service pricing, particularly the average-cost approach. If rates are based upon average or fully allocated cost, it becomes necessary to apportion these cost by some arbitrary means. Average cost pricing with fixed or common cost or both makes these cost price-determining when they should be price-determined. In other words, fixed cost par unit depend on the volume of traffic, and the volume of traffic depends on the rate charged. To some extent then, cost is a function of the prices; the prices are not function of the cost. In fact, it could be argued that not only do costs determine prices, but also that prices determine cost; in other words, the situation is analogous to the chicken and the egg argument.