Table 1

Controlled quality improvement studies—different types of controls take care of different types of confounding

Type of controlDescriptionWhat it controls for (and what not)
Location-based controlsThe control is another location that is similar to the intervention location but has not received the intervention, for example, another ward in the same hospital or another similar hospital.Controls for confounding factors that will affect both locations, but not factors uniquely tied to the intervention location.
Characteristic-based controlsIf the intervention targets specific patient groups defined by their characteristics, for example, elderly, patients with a specific diagnosis, then non-targeted groups may act as controls.Controls for confounding factors that will affect both patient groups, but not factors specific for the intervention group. If the characteristic defining who is intervention/control (eg, age) is a confounder, then this cannot be controlled for.
Historical controlsThis type of control can only be used if such historical data are available (and with sufficient completeness).Potentially, it might help to control for seasonal effects, but cannot control for confounding factors that are specific for the period when the intervention was implemented (eg, changes in case mix).
Outcome controlsThe control is another outcome in the same population not targeted by the intervention (or expected to be not very sensitive for the intervention).Controls for those confounding factors that would affect both the intervention and control outcome, but not those unique for the intervention outcome.