CIE common mistakes
CIE · Understand the traps and how to avoid them.
Applying a blanket allowance percentage to the whole task instead of assigning allowances to the specific elements where they arise (e.g., adding a personal allowance to a machine-paced element).
Candidates memorize a single total allowance number and apply it uniformly, forgetting that allowances attach to elements and that machine-controlled elements generally receive no personal/fatigue allowance.
What to do instead
Build the time standard element-by-element: classify each element as manual, machine, or machine-manual, then apply only the allowances appropriate to that class before summing.
Computing a time standard from an observed average without first adjusting for the operator's performance rating, and confusing normal time with standard time.
The formulas chain together (observed time x rating = normal time; normal time x (1 + allowance) = standard time) and under time pressure candidates skip a step or apply the allowance to the observed time.
What to do instead
Write the two-step chain explicitly on scratch paper for every problem and label each intermediate value (OT, NT, ST) so the units and stage are unambiguous.
Using the wrong sample-size formula for work sampling versus time study, or plugging a proportion into a formula that requires a standard deviation in time units.
Work sampling uses n = z^2 p(1-p)/e^2 (proportions), while continuous time study uses n = (zs/(ex))^2 (time units); candidates mix the two because both are called 'sample size'.
What to do instead
Before computing, ask 'am I estimating a proportion of occurrences or a mean time?' and select the matching formula; memorize the proportion version with p(1-p) as its signature.
Treating a control chart's control limits as specification limits, or concluding a process is capable because all points fall inside the control limits.
Control limits come from the process (mean +/- 3 sigma of the statistic), while specification limits come from the customer; candidates conflate 'in control' with 'capable'.
What to do instead
Separate the two questions: control limits answer 'is the process stable?', capability indices (Cp, Cpk) compare voice of process to voice of customer. Always compute Cpk against spec limits, never against control limits.
Selecting the wrong p-chart versus np-chart versus c-chart versus u-chart for a given defect scenario.
The distinction hinges on whether the subgroup size is constant and whether you count defective units (attribute) or defects per unit; candidates default to the p-chart for everything.
What to do instead
Use a decision tree: constant sample size + fraction defective = p or np; variable sample size + fraction defective = p; constant area/unit + count of defects = c; variable area/unit + count of defects = u.
Miscalculating Economic Order Quantity by using annual demand in one term and monthly demand in another, or forgetting to convert holding cost to a per-unit-per-year basis.
EOQ = sqrt(2DS/H) requires D, S, and H in consistent time units; candidates plug a monthly demand with an annual holding cost, producing an answer off by a factor of sqrt(12).
What to do instead
Before substituting, write the units next to each symbol (D units/year, S cost/order, H cost/unit/year) and convert everything to a single time basis first.
Confusing the critical path with the longest-duration activity, or failing to recompute float after a duration change in a network problem.
The critical path is the longest path through the network (zero total float), not the single longest activity; candidates also reuse original float values after modifying durations.
What to do instead
Forward-pass to get early times, backward-pass to get late times, then compute total float = LS - ES for each activity; the path of zero-float activities is critical. Redo both passes whenever a duration changes.
Ignoring the time value of money by comparing alternatives on simple payback or total undiscounted cost, or mismatching the interest period with the compounding period.
Payback ignores cash flows after the payback point and the timing of money; candidates also apply an annual rate to monthly cash flows without adjusting i and n.
What to do instead
Convert all cash flows to a common point using the effective rate per compounding period; use present worth, annual worth, or IRR for comparison, and reserve payback only as a liquidity screen.
Drawing a from-to chart or relationship diagram without first establishing the flow-volume basis, producing a layout that ignores the dominant material flows.
Candidates jump to layout geometry (block diagrams) before quantifying flow intensities, so high-volume moves get placed far apart.
What to do instead
Sequence the method: part routings -> flow-between chart (quantities) -> relationship chart (qualitative) -> activity relationship diagram -> space requirements -> block layout. Rank departments by flow volume before placing them.
Misapplying therblig classifications, e.g., labeling 'search' or 'find' as an effective (value-adding) element, or treating 'hold' as necessary rather than avoidable.
Candidates memorize therblig names but not the effective/ineffective/avoidable grouping, so method-improvement recommendations target the wrong elements.
What to do instead
Memorize the three classes: effective (reach, move, grasp, release, use), ineffective-but-necessary (hold, position, pre-position), and avoidable (search, find, select, inspect, plan, rest to overcome load). Direct improvement effort at the avoidable class first.