Labor Modeling

Labor is 50-70% of total warehouse operating cost. The difference between a well-run and poorly-run operation is almost entirely explained by labor productivity — not technology. A labor model built on assumed rates is a guess. The cost of measuring correctly is almost always less than the cost of being wrong.

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Process Time Studies: The Foundation

Engineered labor standards (ELS) are scientifically derived time expectations for each task. Every FTE calculation, capacity model, and automation business case depends on them.

Five Methods

Method	Best Use Case	Accuracy
Continuous timing	Long-cycle tasks: forklift putaway, receiving	±10–15%
Snapback timing	Short repetitive tasks: each picking, scan-verify	±8–12%
Predetermined (MTM/MOST)	Pre-launch standards before operation exists	±5%
Work sampling	Indirect labor, travel quantification	±5–10%
WMS data mining	Large-sample validation of existing standards	±3–5%

WMS data mining is the most powerful — thousands of observations without a stopwatch — but only validates what’s already happening, not whether the method is correct.

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Continuous Timing: Step by Step

1. Define task boundaries — clear observable start and end point per element
2. Record cumulative time — never reset the stopwatch; differences = element times
3. Rate worker performance using Westinghouse scale:

Rating	Description
60-70%	Very slow; possible new hire
85-90%	Slightly below normal
100%	Normal pace; trained; sustainable for 8 hours
115-125%	Above normal; brisk
130%+	Exceptional; not sustainable

4. Calculate Normal Time: Normal Time = Observed Time × Performance Rating Factor

Example: 0.45 min observed × 90% rating = 0.405 min Normal Time

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PFD Allowances: Normal Time to Standard Time

PFD = Personal, Fatigue, Delay. The allowance converts Normal Time into Standard Time — what a trained worker delivers across an actual shift.

Formula: Standard Time = Normal Time × (1 + PFD%)

Component	Typical %	What It Covers
Personal	5%	Restroom, water, brief personal
Fatigue	5-15%	Physical exertion; light = 5%, heavy lift = 10-20%
Delay	5%	Equipment issues, WMS delays, waiting
Total — light warehouse	15-20%	Under-30 lb items
Total — freezer/cooler	20-25%	Cold adds meaningful fatigue
Total — heavy lift	20-25%	Cases over 50 lbs

Rule of thumb: 15% PFD for standard picking/packing. 20% for freezer or heavy case operations.

Worked example:

Scheduled breaks: 2 × 12 min + 35 min other lost time = 59 min
Shift: 480 min
PFD = 480 ÷ (480 - 59) - 1 = 14.0%

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Sample Size: How Many Observations

20 observations is preliminary — adequate to identify obvious problems, not statistically valid for labor standards driving FTE funding.

Formula: n = (z × s / (e × x̄))²

Where z = 1.645 (90% confidence) or 1.96 (95%), s = standard deviation, e = 0.05 (±5% error), x̄ = mean.

Practical minimums by task type:

Task Type	Minimum Observations
Short-cycle repetitive (<30 sec)	50-100
Medium-cycle (30 sec - 2 min)	30-50
Long-cycle (2-10 min)	20-30
Very long-cycle or infrequent (>10 min)	10-20 (supplement with MTM)

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WMS Data Mining

Every WMS transaction leaves a timestamp. Step-by-step:

1. Pull TASK, TASK_DETAIL, USER_LOG records
2. Filter to trained workers only (30+ days, 500+ task completions)
3. Calculate Cycle Time = End Timestamp - Start Timestamp
4. Remove outliers (< 5 seconds or > 3 standard deviations)
5. Use median (not mean) for standard setting
6. Convert to throughput: Rate (units/hr) = 3,600 ÷ Median Cycle Time (seconds)
7. Apply PFD correction: Standard Rate = WMS Rate ÷ (1 + PFD%)

WMS-derived rates already embed some real-world delays — discuss with your team whether to apply full or reduced PFD.

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Fully-Burdened Labor Rate

The hourly wage is only part of the cost. Always use the fully-burdened rate:

Base Wage	Typical Burden %	Burdened Rate
$15/hr	35-45%	$20-22/hr
$18/hr	32-42%	$24-26/hr
$22/hr	30-38%	$29-31/hr
$25/hr	28-35%	$32-34/hr

Burden includes FICA (7.65%), FUTA, SUI (1-4%), workers’ comp (2-6%), health insurance ($3K-8K/yr employer share), PTO, 401(k) match, and recruiting costs ($1,500-4,000 per hire amortized).

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Staffing Model Framework

Step 1: Peak throughput (size to design day, not average)

Step 2: Apply efficiency factor:

• New/training workers: 60-70% of standard
• Experienced workers: 85-95% of standard
• Model recommendation: design to 85%

Step 3: Required headcount = Peak throughput ÷ (Standard × Efficiency factor)

Step 4: Add indirect labor:

• Inbound/receiving/putaway: 20-30% of direct labor headcount
• Supervisors: 1 per 10-15 direct workers

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Common Modeling Errors

• Assumed rates from trade shows or vendor decks instead of measured standards. A 40% understaffed DC is the typical outcome.
• Learning curve contamination — studying workers who are 3 weeks on the job. Wait until the plateau: 4-8 weeks for complex picking, 2-3 weeks for scan-and-pack.
• Performance rating bias — analysts new to time studies rate too fast. Use video calibration sets before field studies.
• Outlier contamination — apply 3-sigma rule; flag elements >3 standard deviations as abnormal events.