Industrial FE Exam: Common Mistakes

Use the z-distribution when the population standard deviation is known and the sample is large (n > 30). Use the t-distribution when the population standard deviation is unknown. Use chi-square for variance testing and F-distribution for comparing two variances. Choosing the wrong distribution changes your critical values and your conclusion.

Resource constraints use less-than-or-equal-to inequalities (you cannot exceed available resources). Demand or minimum requirements use greater-than-or-equal-to inequalities. A flipped inequality changes the feasible region entirely, producing an optimal solution that violates the actual problem constraints.

Use X-bar and R charts for variable data measured on a continuous scale. Use p-charts for proportion defective and c-charts for count of defects. Applying a variables chart to attribute data (or vice versa) makes the control limits meaningless and leads to false alarms or missed signals.

Never compare cash flows occurring at different times without discounting them to a common point. All alternatives must be compared at the same point in time using present worth, annual worth, or future worth analysis. Simply adding up undiscounted costs and revenues over time ignores interest and inflation effects.

Cp measures potential capability assuming the process is centered, while Cpk accounts for how far the process mean has shifted from center. A process can have a high Cp but low Cpk if the mean is off-center. Always calculate both and report Cpk as the true measure of how well the process meets specifications.