Tuy nhiên, sự phụ thuộc tiêu cực được đưa vào các mối quan hệ thời gian hoạt động lập kế hoạch dự phòng gây ra sự phụ thuộc mạnh mẽ, tích cực giữa các chi phí liên quan. Nếu A chi phí nhiều hơn dự kiến , | 146 Structure the issues However the negative dependence introduced into the activity duration relationships by contingency planning induces strong positive dependence between associated costs. If A costs more than expected B tends to cost very much more than expected because of the need to keep the project on target quite apart from other market-driven sources of dependence. Put another way cost and duration modelling of uncertainty that does not explicitly consider contingency planning tends to estimate time uncertainty erroneously usually optimistically and fails to structure or explain it and tends grossly to underestimate direct cost uncertainty. Considering the impact of contingency planning will clarify apparent time uncertainty and increase apparent direct cost uncertainty. Common causes of knock-on effects are design changes and delays which not only have a direct impact but also cause ripple effects termed delay and disruption . Often direct consequences can be assessed fairly readily in terms such as the number of man-hours required to make a change in design drawings and the man-hours needed to implement the immediate change in the project works. Ripple effects are more difficult to assess and may involve snowballing effects such as altered work sequences conflicting facility and manpower requirements skill dilution undetected work errors and so on. Example Widening fire doors causes substantial delays In 1991 apparently small changes in the design of fire doors on Channel Tunnel rolling stock was expected to lead to a delay of up to six months in providing a full service for car and coach passengers substantially reducing expected revenue for Eurotunnel operators of the tunnel. The problem was caused by the insistence of British and French authorities that the width of the fire doors separating the double-deck car shuttles should be widened from 28 to 32 inches Taylor 1991 . Example A delay-and-disruption claim Cooper 1980 has described how a .
