Lean Systems and Process Improvement
Lean thinking focuses on eliminating waste (muda) to create flow and pull. Candidates must recognize the seven wastes: overproduction, waiting, transportation, overprocessing, inventory, motion, and defects (some lists add unused talent). Core lean tools include value stream mapping (current and future state), 5S (sort, set in order, shine, standardize, sustain), kanban pull systems, cellular manufacturing, SMED (single-minute exchange of die) for setup reduction, and standard work.
Continuous improvement frameworks: PDCA (plan-do-check-act), DMAIC (define, measure, analyze, improve, control) for Six Sigma, kaizen events, and A3 problem solving. Root-cause tools include the 5 Whys, fishbone (Ishikawa) diagrams, and Pareto analysis. Six Sigma metrics: sigma level relates to defects per million opportunities (DPMO); 3.4 DPMO corresponds to 6 sigma. Traps: confusing muda (waste) with mura (unevenness) and muri (overburden); treating kanban as a scheduling system rather than a pull signal; and assuming 5S is only housekeeping rather than a foundation for standardized work.
Little's law is frequently tested: inventory (WIP) = throughput × flow time (I = R × T). It applies to any stable system and lets you compute WIP, throughput, or lead time from the other two. Takt time = available time / customer demand sets the pace. Value-added ratio = value-added time / total lead time; most processes have very low ratios, and the improvement target is reducing non-value-added time.
Worked example
A process has throughput 20 units/hr and flow time 3 hr. WIP = 20 × 3 = 60 units. If demand is 160 units over an 8-hr shift, takt = 480/160 = 3 min/unit; the line must produce one unit every 3 minutes to meet demand.