🎓 Lesson 4
D3
Stopwatch Time Study: Best Practices & Pitfalls
A stopwatch time study is a method where you use a stopwatch to measure how long workers take to complete specific tasks on the shop floor, so you can improve efficiency and fairness.
🎯 Learning Objectives
- ✓ Calculate observed time, normal time, and standard time from raw stopwatch data
- ✓ Apply allowance factors (personal, fatigue, delay) to derive realistic standard times
- ✓ Analyze time study data to identify non-value-added elements and recommend process improvements
- ✓ Design a valid time study plan—including cycle count, sampling strategy, and observer protocol—per industry best practices
- ✓ Explain the impact of observer bias, worker awareness, and environmental variability on time study validity
📖 Why This Matters
In mining and blasting operations, labor efficiency directly affects cost per ton, safety compliance, and schedule adherence. A poorly conducted time study can misallocate crews, underfund training, or incentivize unsafe shortcuts—while a rigorous one enables fair pay structures, accurate fleet dispatching, and reliable mine planning. For example, miscalculating drill rig crew cycle time by just 8% can compound into >200 hours of annual overstaffing across a 12-rig fleet.
📘 Core Principles
Stopwatch time study rests on three pillars: (1) Task decomposition—breaking work into observable, measurable elements (e.g., 'position drill rig', 'set up collar', 'drill hole'); (2) Representative sampling—collecting enough cycles to achieve statistical confidence (typically ≥10–20 cycles per element, per ANSI/ISO standards); and (3) Performance rating—objectively assessing worker pace relative to a defined 'normal' pace (e.g., 100% = trained, motivated worker using proper method). Critical nuances include distinguishing between constant and variable elements, handling interruptions (e.g., equipment delays), and recognizing when stopwatch methods are inappropriate (e.g., highly irregular, infrequent tasks better suited to work sampling).
📐 Standard Time Calculation
Standard time integrates observed time, performance rating, and allowances to reflect realistic, sustainable work pace. It serves as the basis for labor planning, costing, and KPI tracking in mining operations.
Standard Time (ST)
ST = (OT × PR) / (1 − A)Computes the time required for an average worker to complete a task at standard performance, including allowances for rest and delays.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| OT | Observed Time | minutes or seconds | Average measured time per cycle, excluding outliers and interruptions |
| PR | Performance Rating | decimal (e.g., 0.95) | Rater's judgment of worker pace relative to standard (100% = standard pace) |
| A | Total Allowance Factor | decimal (e.g., 0.15) | Sum of personal, fatigue, and delay allowances expressed as fraction of total time |
Typical Ranges:
Drill rig setup (surface): 3.5 – 6.2 min
Blast hole stemming (manual): 1.8 – 3.0 min
Shovel loading cycle (large hydraulic): 4.5 – 5.8 min
💡 Worked Example
Problem: An observer records 15 cycles of 'load muck onto haul truck' with an average observed time of 4.2 min/cycle. The worker’s pace is rated at 95% (0.95). Allowances are: personal (5%), fatigue (8%), and unavoidable delay (2%).
1.
Step 1: Calculate Normal Time = Observed Time × Performance Rating = 4.2 min × 0.95 = 3.99 min
2.
Step 2: Sum allowances = 5% + 8% + 2% = 15% = 0.15
3.
Step 3: Apply allowance factor: ST = Normal Time / (1 − Total Allowance) = 3.99 / (1 − 0.15) = 3.99 / 0.85
4.
Step 4: Compute result: 3.99 ÷ 0.85 = 4.694 min ≈ 4.70 min
Answer:
The standard time is 4.70 minutes per cycle, which falls within the typical range of 4.2–5.5 minutes for this task in hard-rock underground loading operations.
🏗️ Real-World Application
At Newmont’s Boddington Mine (Western Australia), engineers conducted a stopwatch time study on shovel-truck loading cycles to validate fleet simulation inputs. They observed 22 cycles per operator shift across 3 shifts, applied video-assisted rating to minimize observer bias, and incorporated GPS-derived delay logs to separate productive vs. waiting time. The revised standard time (5.12 min/load) replaced a legacy value of 4.68 min—correcting a 9% overestimation of shovel productivity that had skewed truck allocation models for 18 months. Post-implementation, haul truck utilization improved by 12%, reducing diesel consumption per ton by 3.4%.
✏️ Your Turn: Analyze & Adjust
You observe 12 cycles of 'connect detonation cord to blast hole' with times (in seconds): 28, 31, 27, 33, 29, 30, 26, 32, 28, 31, 29, 30. Worker is rated at 105%. Allowances total 18%. Calculate: (a) average observed time, (b) normal time, (c) standard time. Then explain whether this task likely requires additional elemental breakdown—and why.
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