Q.Distinguish between chance (random) causes and assignable causes of variation in a manufacturing process, with one example of each.

The distinction is really about size and traceability — one kind of variation is small, everywhere, and unavoidable, while the other is large, specific, and fixable.

Chance (random) causes are the many small, individually unidentifiable influences that are always present in any manufacturing process — tiny differences in raw material, slight environmental changes, the natural variability of a machine or an operator's hand movement. No single chance cause can be traced or removed, but together they produce a stable, predictable amount of natural variation. A process affected only by chance causes is said to be in (statistical) control. Example: tiny, millimetre-level differences in the diameter of machine-turned bolts even when the lathe is working correctly.

Assignable causes are large, specific, identifiable sources of variation that are not part of the process's normal behaviour — a worn-out or wrongly-set tool, a defective batch of raw material, an untrained operator, a machine that has slipped out of calibration. Because they can be traced to a specific, findable cause, they can (and should) be corrected. A process affected by an assignable cause is said to have gone out of control. Example: a cutting tool that has become badly worn, causing every subsequent item's dimension to drift noticeably larger than before. …

Treating any variation at all as a sign something is wrong (some variation — chance-cause variation — is normal and expected in every real process); and giving an assignable-cause example that is actually too small/uniden …