Project Scheduling and Cost Control
Schedule and cost as one system
The critical path method is the backbone: activities, durations, logic ties (finish-to-start, start-to-start, finish-to-finish) with lags, and a forward and backward pass producing early and late dates. Total float is the gap between late and early dates; free float is how far an activity can slip without delaying its immediate successor. Only zero-total-float activities are critical, and the critical path is the longest path - not necessarily the one with the most work. Know crashing (adding resources; costs money) versus fast-tracking (overlapping phases; adds risk), and how resource leveling rewrites logic and durations.
Cost control
Understand job cost coding, committed versus incurred cost, buyout and savings, and earned value. Planned value (PV), earned value (EV), and actual cost (AC) drive SPI = EV ÷ PV and CPI = EV ÷ AC. An SPI below 1 means behind schedule; a CPI below 1 means over budget. Read trends, not single points, and remember earned value measures progress in dollars, not calendar time.
Traps
Frequent errors: treating float as a contractor entitlement when the contract assigns it to the project; assuming all float belongs to one trade; and "updating" a schedule without revising logic, which hides the true cause of delay. Delay questions usually hinge on contemporaneous updates and on which party owned the float being consumed.
Worked example
PV 500,000; EV 400,000; AC 450,000. SPI = 0.80 - about 20 percent behind plan. CPI = 400,000 ÷ 450,000 = 0.89 - spending more than earned. Forecast cost at completion by dividing budget by CPI.