How to choose an inspection level: I, II, III and S-1 to S-4

The inspection level decides how large the sample is for a given lot. It does not change the AQL. This page sets out what MIL-STD-105E says about the seven levels and shows the sample sizes they lead to.

What the level does

Clause 4.9.1 of MIL-STD-105E: the inspection level determines the relationship between the lot or batch size and the sample size.

In practice the level picks the column of the code letter table. The letter then picks the row of the sampling table, and the row carries the sample size.

The three general levels

The same clause gives three levels for general use and says how they relate:

• Level II is the one normally used.
• Level I may be used when less discrimination is needed.
• Level III may be used for greater discrimination.

The four special levels

Clause 4.9.1 also gives four special levels, S-1, S-2, S-3 and S-4. They may be used where relatively small sample sizes are necessary and large sampling risks can or must be tolerated.

Sample sizes side by side

Code letter and sample size for three lot sizes, from Table I and the sample size column of Table II-A:

Level	Lot of 500	Lot of 5,000	Lot of 50,000
S-1	B, 3	C, 5	D, 8
S-2	C, 5	D, 8	E, 13
S-3	D, 8	F, 20	G, 32
S-4	E, 13	G, 32	J, 80
I	F, 20	J, 80	L, 200
II	H, 50	L, 200	N, 500
III	J, 80	M, 315	P, 800

These are the sample sizes that belong to each letter. The sample actually used can differ once the AQL is applied, because some cells of the table hold an arrow that leads to another letter.

The level has to fit the AQL

The note to clause 4.9.1 warns that, in selecting levels S-1 to S-4, care must be exercised to avoid AQLs inconsistent with these levels. The purpose of the special levels is to keep samples small when necessary.

The standard gives its own example: the code letters under S-1 go no further than D, equivalent to a single sample of size 8, but it is of no use to choose S-1 if the AQL is 0.10 percent, for which the minimum sample is 125.

The table shows why. A lot of 5,000 at S-1 gives letter C, with a sample size of 5. In the 0.10 column the cell for C is a down arrow, and the first plan below it belongs to letter K: 125 units, Ac 0, Re 1. The small sample that S-1 was chosen for is gone.

Who sets the level

The standard does not pick a level for you. Clause 4.9.1 says the level to be used for any particular requirement will be as prescribed by the written procedures. The level, like the AQL, has to be settled and written down before inspection starts.

Level is not the same as normal, tightened or reduced

These are two different settings.

• The inspection level (S-1 to S-4, I, II, III) is chosen once and fixes the code letter.
• Normal, tightened and reduced inspection change with results. Clause 4.6 says normal inspection is used at the start. Clause 4.7.1 moves to tightened inspection when 2 out of 5 or fewer consecutive lots have been rejected on original inspection, and clause 4.7.2 returns to normal when 5 consecutive lots have been accepted. Clause 4.7.3 lists the conditions for reduced inspection.

The tables on this site are for normal inspection only.

Questions people ask

Which inspection level is the default?

Clause 4.9.1 says that normally inspection level II is used.

Does a higher level mean a stricter AQL?

No. The level changes the sample size; the AQL is set separately. Clause 4.9.1 describes level III as giving greater discrimination and level I as giving less.

When are the special levels used?

Clause 4.9.1 says they may be used where relatively small sample sizes are necessary and large sampling risks can or must be tolerated.

Source

• MIL-STD-105E, Sampling Procedures and Tables for Inspection by Attributes, U.S. Department of Defense, 10 May 1989: Table I, Table II-A, and clauses 4.6, 4.7.1, 4.7.2, 4.7.3 and 4.9.1 with its note. Page scans: archive.org/details/MIL-STD-105E_1. Read 11 October 2026.

Where your contract names ANSI/ASQ Z1.4, ISO 2859-1 or another standard, use the tables and wording of that edition.