Choosing Between Minitab Assistant and Manual 2‑Sample t‑Test for Process Mean Comparisons

The problem: deciding whether two independent processes differ

You have two sets of measurements — perhaps cycle times from line A and line B — and you need a statistically sound answer to “are the means different?” Minitab offers two entry points for the same 2‑sample t‑test: the guided Assistant and the classic Stat → Basic Statistics dialog. Both produce a p‑value and confidence interval, but they differ in assumption reporting, default options, and how they handle version or license changes.

When the Assistant shines

The Assistant (Assistant → Hypothesis Tests → 2‑Sample t‑test) is built for engineers who want a quick “report card” without digging into dialog boxes. It automatically:

• Checks normality for each group (Anderson‑Darling test and normal probability plot).
• Flags outliers with a boxplot and a list of suspect rows.
• Warns if the data appear paired or severely skewed.
• Summarizes practical significance with a default “meaningful difference” threshold.

All of this appears in a single Summary Report plus a Report Card that rates each assumption as “OK”, “Caution”, or “Fail”. For a first pass, this is often enough to decide whether to trust the p‑value.

When you need the manual dialog

Stat → Basic Statistics → 2‑Sample t gives you full control:

• Choose equal or unequal variances (Welch’s correction).
• Set any confidence level (e.g., 99 % for high‑risk decisions).
• Specify a one‑sided alternative if the engineering question is directional.
• Force the test to treat data as stacked (one column + grouping column) or unstacked (two separate columns).

Use this path when the Assistant’s default “practical significance” does not match your tolerance, cost, or risk criteria, or when you must document exact settings for a regulated procedure.

Worked example: two columns vs. stacked layout

Assume you have 15 measurements from each line. In the worksheet you can arrange them either way:

// Unstacked (two columns)
C1  LineA   C2  LineB
12.3        13.1
11.9        13.4
...         ...

// Stacked (one column + grouping)
C1  Time   C2  Line
12.3       A
13.1       B
11.9       A
13.4       B
...        ...

Assistant path (run on a licensed Minitab 20/21 session):

1. Open the worksheet with the unstacked layout.
2. Choose Assistant → Hypothesis Tests → 2‑Sample t‑test.
3. Select “Samples in different columns”, pick C1 and C2.
4. Click OK. The Summary Report shows the p‑value, 95 % CI, and a Report Card with normality, outlier, and practical‑significance rows.

Manual path on the same data:

1. Choose Stat → Basic Statistics → 2‑Sample t.
2. Select “Each sample in its own column”, choose C1 and C2.
3. Click Options → set Confidence level to 95, Alternative hypothesis to “Difference ≠ 0”, and Assume equal variances unchecked (Welch).
4. Click OK. Compare the p‑value and CI with the Assistant output — they should match because the underlying test is identical.

Verification tip: After running both, open Help → About Minitab and note the version (e.g., 21.4) and edition (Professional, Quality Companion, etc.). Menu locations and default “practical significance” wording can shift between releases, so record the exact path for your team’s SOP.

Trade‑offs and limitations

• Practical significance ≠ engineering tolerance. The Assistant suggests a “meaningful difference” based on a generic effect‑size rule; you must replace it with your own spec limit.
• Small samples weaken assumption checks. With n < 15 per group, normality tests have low power and outlier flags can be noisy.
• Dialog drift across versions. In Minitab 19 the Assistant lived under Assistant → Hypothesis Tests; in 20/21 it moved to Assistant → Hypothesis Tests → 2‑Sample t‑test. Always confirm on the installed build.
• License gating. The Assistant is only available in editions that include the “Assistant” module (e.g., Professional, Quality Companion). The basic Stat dialog is present in all editions.

Actionable next step

Run both workflows on a representative data set from your process. Capture the Assistant Report Card and the manual dialog settings (screenshot or export). Document the exact menu sequence for your Minitab version and edition in your analysis protocol. This gives you a repeatable, auditable decision trail and lets you switch to the manual dialog instantly when the Assistant’s defaults no longer fit the engineering question.