power in off-peak hours or going to the theater mid-week), their demand sensitivity (must have the higher cost early morning flight or must go to the Saturday night opera) or their time of purchase (usually paying a premium for booking late). In this way, yield management's overall aim is to provide an optimal mix of goods at a variety of price points at different points in time or for different baskets of features. The system will try to maintain a distribution of purchases over time that is balanced as well as high. Good yield management maximizes (or at least significantly increases) revenue production for the same number of units, by taking advantage of the forecast of high demand/low demand periods, effectively shifting demand from high demand periods to low demand periods and by charging a premium for late bookings. While yield management systems tend to generate higher revenues, the revenue streams tends to arrive later in the booking horizon as more capacity is held for late sale at premium prices. Firms faced with lack of pricing power sometimes turn to yield management as a last resort. After a year or two using yield management, many of them are surprised to discover they have actually lowered prices for the majority of their opera seats or hotel rooms or other products. That is, they offer far higher discounts more frequently for off-peak times, while raising prices only marginally for peak times, resulting in higher revenue overall. By doing this, they have actually increased quantity demanded by selectively introducing many more price points, as they learn about and react to the diversity of interests and purchase drivers of their customers. Ethical issues and questions of efficacy Some consumers are concerned that yield management could penalize them for conditions which cannot be helped and are unethical to penalize. For example, the formulas, algorithms, and neural networks that determine airline ticket prices could feasibly consider frequent flyer information, which includes a wealth of socio-economic information such as age and home address. The airline then could charge higher prices to consumers who are between certain ages or who live in neighborhoods with higher average wealth, even if those neighborhoods also include poor households. Very few (if any) airlines using yield management are able to employ this level of price discrimination because prices are not set based on characteristics of the purchaser, which are in any case