Man-day calculation logic (batch 2 guide 11)

Key takeawaysMan-days follow content: SKUs, sample sizes, tests, and sites, not order value or carton counts alone. · Sampling logic sets the floor: larger lots and tighter limits mean larger samples and longer checks. Acceptance sampling principles explain how a defined sample can represent a larger lot, which is why systematic carton selection matters more than convenience picks ^[1] · Each extra style repeats the full selection, measurement, and recording cycle. · On-site tests and travel convert directly into hours that must appear in the estimate. RoHS restricts defined hazardous substances in electrical and electronic products, so material declarations and component markings deserve attention even at an early stage ^[2] · A stated assumption sheet lets both sides adjust scope or days before the visit, not during it.

SKU variety multiplies the cycle because sampling, measurement, and labeling verification repeat per style. A single-style lot of five thousand pieces often consumes less time than five styles of one thousand pieces each, even though the totals match. This is the single most misunderstood driver in man-day disputes.

An inspector's day divides into selection, checking, recording, and movement. Selection means drawing cartons and units without bias across the presented quantity. Checking means visual, dimensional, and functional work per the checklist. Recording means photos, defect coding, and measurements that make the report defensible. Movement means walking lines, warehouses, and between sites.

Sampling plans translate lot size into sample size through defined tables and switch rules, so the checking block can be reasoned about in advance. More units in the sample means more handling minutes; tighter acceptance numbers mean closer attention per unit. Neither is negotiable on the day without changing the integrity of the result.

Where inspector hours go

Start the estimate from the sample math. For each style, note the presented quantity, derive the sample size under the agreed plan, and multiply by a per-unit checking allowance that reflects the product: simple soft goods run faster per unit than wired electro-mechanical items with power-on steps. Summing across styles gives the core checking hours.

Schedule the pre-production inspection around open production lines rather than finished-goods cartons alone, because corrections cost less while the line is still running.

Layer the test list on top as discrete time blocks. Dimensional sets, function sequences, shade evaluations, and drop or assembly verifications each carry setup plus per-unit minutes. Buyers who write tests as named lines with quantities get honest totals; buyers who write test everything get optimistic ones.

From samples to a day count

Add recording time proportional to variety. Each style needs its own photo series, measurement table, and defect log entries, and mixed findings need more annotation than clean runs. Under-budgeted recording is why some reports arrive thin: the work was done, but the evidence was compressed to fit the remaining hour.

Add movement honestly, including within-site walking. Large campuses with separate packing, storage, and loading zones consume real minutes between draws. Multi-site routes add inter-city travel that can only be shared when geography allows; plotting addresses on a map before estimating prevents the classic one-day plan for a two-city job. Acceptance sampling principles explain how a defined sample can represent a larger lot, which is why systematic carton selection matters more than convenience picks ^[1]

Apply a complexity factor for new suppliers, new products, or unstable processes. First visits need longer opening meetings, document review, and calibration alignment. A modest uplift here is not padding; it reflects the real cost of unfamiliarity that repeat visits gradually repay.

Time blocks and misses

An estimate that names every row can be debated and fixed; one that hides rows can only be regretted.

Estimate in five passes

1. Fix the presented quantityConfirm per-style quantities and carton counts with the factory so sample sizes rest on numbers, not hopes.
2. Derive samples per styleApply the agreed sampling plan style by style; keep mixed styles as separate lines throughout.
3. Price the test listAttach minutes to each named test, including setup, and sum them as visible blocks in the estimate.
4. Add recording and movementAllow photo, table, and walking time from campus and route facts, then round to days with the rounding stated.
5. Publish assumptionsSend the one-page math with the booking so scope changes trigger day adjustments before mobilization.

Calibrate and protect the math

Use past jobs as calibration data. After each visit, compare the planned hours per block against actuals and adjust the per-unit allowances for that product family. Three cycles of this feedback usually stabilize estimates to within a half-day for repeat assortments.

Resist compressing the estimate to fit a preferred date. A one-day booking for a demonstrably two-day scope does not create efficiency; it creates skipped tests and thin photos. Better to split across two inspectors in one day or two consecutive days than to pretend the arithmetic is wrong.

Coverage notes for this job: Acceptance sampling principles explain how a defined sample can represent a larger lot, which is why systematic carton selection matters more than convenience picks ^[1].

Estimation note

Frequently asked questions