Although the engineering cost estimating technique avoids some of the pitfalls of regression analysis, it suffers from some problems of its own. First, the estimates represent what engineers and cost estimators believe cost should be, not necessarily what they actually will be. Because they are really forecasts, calculations may omit certain components that contribute to costs. Further, most of the time, only direct output costs are estimated. Other costs that may be directly associated with the product ( e.g., some portion of overhead and direct selling expenses) are not included, or if they are, it is though some rather arbitrary allocation. Often such estimates are made on the basis of pilot plant operations and do not consider actual production, which may be attended by bottlenecks and other problems, causing costs to differ from those estimated.