CMA Inter Paper 9: OMSM common mistakes
CMA Inter Paper 9: OMSM · Understand the traps and how to avoid them.
Treating productivity as a single formula and computing total productivity when the question asks for partial (single-factor) productivity, or mixing units (rupees of output over labour hours) without converting to a common base.
Candidates memorise 'Productivity = Output/Input' and then apply whichever input figure appears first in the data, ignoring whether the question specifies labour, material, machine or total factor productivity. Unit mismatches then go unnoticed because the arithmetic still produces a number.
What to do instead
Before computing, write down which input the question names and label the answer with its unit (e.g., units per labour hour, or rupees of output per rupee of input). If total productivity is required, convert every input to a common monetary value first. Practise a three-input example where partial and total productivity give different rankings.
Confusing the four process strategies (project, job shop, batch, mass/line, continuous) when classifying a described operation, or matching a layout type (process, product, fixed-position, cellular) to the wrong process strategy.
The strategies sit on a continuum of volume and variety, and candidates classify by industry label ('hospital = service') rather than by volume-variety characteristics. Layout and process strategy are studied separately, so the linkage is missed.
What to do instead
Build a two-axis mental map: volume on one axis, variety on the other. Place each strategy on it, then note which layout fits each quadrant (job shop to process layout, mass to product layout, project to fixed-position, group technology to cellular). Test yourself by classifying ten unfamiliar operations by volume and variety, not by sector.
Applying a fixed-order-quantity model when the item is better suited to a fixed-time-period model, or using the EOQ formula without checking whether quantity discounts or stock-out costs change the decision.
EOQ is the most drilled formula, so it becomes the default. Candidates also forget that EOQ minimises ordering plus carrying cost only, and ignore purchase cost when discounts apply.
What to do instead
State the assumptions of EOQ (constant demand, no shortages, fixed lead time, no discounts) before using it. When discounts appear, compute total cost (purchase + ordering + carrying) at each price break and compare with the EOQ total. When demand is lumpy or orders must be periodic, use the fixed-period model. Always sanity-check the answer against the data.
Miscalculating crashing decisions in project management by crashing the cheapest activity overall rather than the cheapest activity on the critical path, or failing to recompute the critical path after each crash.
Candidates find the critical path once and then treat it as fixed. They also confuse crash cost per unit time with total crash cost, and forget that a non-critical activity can become critical after crashing.
What to do instead
Work in a table: activity, normal time, crash time, cost slope, and slack. At each step crash the activity with the lowest cost slope that lies on the current critical path, limited by the smallest slack of parallel paths. Redraw or recompute the network after every crash and stop when the target duration or the point where crashing no longer reduces duration is reached.
Treating quality-control tools interchangeably, e.g., using a Pareto chart where a cause-and-effect diagram is needed, or describing control charts without distinguishing variable from attribute charts or explaining control limits versus specification limits.
The seven tools are listed together in most syllabi, so candidates learn names without use cases. Control limits and specification limits both look like 'limits' on a chart, and the distinction between common and special cause variation is under-practised.
What to do instead
Attach one sentence to each tool describing the question it answers (Pareto: which few causes dominate; cause-and-effect: what could cause this effect; control chart: is the process stable). Memorise that control limits come from process data (mean plus or minus three sigma) while specification limits come from the customer. Practise interpreting an out-of-control pattern such as a run of seven points on one side of the mean.
Confusing the five lean principles with the seven wastes, or listing tools (kanban, 5S, poka-yoke, JIT) without linking each to the waste it removes.
Both lists are short and memorable, so they blur together under exam pressure. Candidates reproduce the vocabulary but cannot apply it to a described process.
What to do instead
Learn the five principles as a sequence (value, value stream, flow, pull, perfection) and the wastes by their common names (overproduction, waiting, transport, over-processing, inventory, motion, defects). For each tool, write the waste it targets: kanban controls overproduction, poka-yoke prevents defects, 5S reduces motion and waiting. Then practise spotting wastes in a short process description.
Answering strategy questions with generic definitions instead of applying a named framework (PESTLE, Porter's Five Forces, SWOT, value chain, BCG) to the facts given in the scenario, and failing to move from analysis to a justified choice.
Candidates memorise framework headings but not how to use them as analytical instruments. Under time pressure they write textbook descriptions of the framework rather than using it to reach a recommendation.
What to do instead
For every framework, practise a one-page application to a described firm: list only factors that matter, state the implication of each, then convert to a strategic option. Always finish with an explicit recommendation and the criterion used (fit with resources, competitive advantage, feasibility). In SWOT, insist that strengths and weaknesses are internal and opportunities and threats external.
Treating strategy formulation and strategy implementation as the same stage, so answers on implementation describe what the strategy is rather than how it will be executed through structure, systems, culture and resources.
Formulation is more analytical and easier to write about, so candidates drift into it. Implementation content (organisational structure, resource allocation, change management, leadership) is less formulaic and gets skipped in revision.
What to do instead
Keep a two-column template: formulation decisions (mission, objectives, choice of strategy) versus implementation levers (structure, systems, culture, people, resource allocation). For any implementation question, name at least three levers and explain how each must change to fit the chosen strategy, plus one barrier to change and how to overcome it.
Describing evaluation and control as a single step ('review performance') without specifying the control process, the standards used, or the corrective action triggered when a variance appears.
Control is treated as an afterthought at the end of the syllabus. Candidates do not practise the sequence of setting standards, measuring performance, comparing against standards and taking corrective action, so their answers stay abstract.
What to do instead
Memorise the four-step control cycle and apply it to a concrete example with numbers: standard, actual, variance, cause, corrective action. Distinguish feedforward, concurrent and feedback control and give one example of each. Link control back to strategy by noting which critical success factors the standards should track.
Writing supply-chain answers that stop at definitions of logistics, procurement and inventory, without addressing integration, the bullwhip effect, supplier selection criteria or the trade-off between responsiveness and efficiency.
The topic spans several chapters and candidates revise it as isolated definitions. They also avoid quantitative parts such as safety stock or reorder point because these feel like operations-management content rather than supply-chain content.
What to do instead
Revise supply chain as a set of flows (material, information, cash) and a set of decisions (facility, inventory, transportation, sourcing, pricing). Practise explaining the bullwhip effect with its causes and remedies, list supplier selection criteria with weighting logic, and work two numerical problems on reorder point and safety stock so the quantitative side is not a blind spot.