critical process parameter
When you bake bread, dozens of settings exist — but oven temperature and proving time are the dials that genuinely decide whether the loaf rises or burns; the colour of your mixing bowl does not. A critical process parameter is one of those decisive dials in manufacturing: a process variable whose variation has a meaningful effect on a critical quality attribute, and which therefore must be monitored and controlled to keep the product within spec.
Defined in ICH Q8, CPPs are identified by studying how changing each parameter affects the product. In granulation that might be the spray rate or amount of binder liquid; in drying, the inlet air temperature and endpoint moisture; in tableting, the compression force and turret speed; in coating, the spray rate and bed temperature. Each critical parameter gets a controlled operating range, and that set of ranges and controls is the heart of the control strategy.
The label critical is relative and earned through data: a parameter that strongly affects a CQA over its normal range is critical, while one with little effect across realistic variation is not, and may be left as a normal (non-critical) parameter. Two honest caveats: parameters interact (compression force and granule moisture together affect hardness, not separately), and a parameter judged non-critical at lab scale can become critical at production scale, which is why scale-up so often reopens these questions.
In tablet compression, the main compression force is a critical process parameter: too low gives soft, friable tablets, too high can slow disintegration and dissolution — so it is held within a validated range and monitored on the press.
Each CPP is bounded by an operating range chosen to keep CQAs in spec.
Material attributes (such as raw-material particle size) sit alongside process parameters; QbD often treats both together as critical material attributes and CPPs feeding into the same quality attributes.