design space
Picture a map of all the combinations of, say, drying temperature and granulation water you could use. Some combinations make good tablets; most do not. The design space is the safe region on that map: the multidimensional combination and interaction of input variables and process parameters that has been demonstrated to provide assurance of quality. Stay inside it and you are confident the product will be good.
Defined in ICH Q8, the design space is established through QbD experiments and is proposed by the applicant and reviewed and approved by regulators as part of the marketing authorisation. Its great practical value is regulatory: working within the approved design space is not considered a change, so a manufacturer can adjust parameters inside it to respond to raw-material variability without filing a formal change and waiting for re-approval.
Two cautions keep this honest. First, a design space is only valid over the ranges actually studied; the boundary is the edge of demonstrated knowledge, not a wall you have proven the far side of, so moving outside it is a change that needs evaluation. Second, a design space is usually scale- and equipment-dependent — one established at pilot scale may not transfer unchanged to a different production line, which must be verified.
A granulation design space might be defined as binder spray rate of 80–120 g/min combined with inlet air 55–70 °C; any pairing inside that region has been shown to yield granules that compress into in-spec tablets.
A design space is a region of jointly varying parameters, not a single set point.
Within the design space there is often a smaller normal operating range — the routine target window — leaving deliberate room between everyday operation and the edge of failure.