Table 2.

The identified measurement issues

Measurement issue	Definition	Number of studies
Incremental informal care time	(i) The amount of total caregiving time that is due to the needs of a particular health condition of a recipient. (ii) The amount of time that is classified as caregiving	16 [23, 28, 29, 38, 40, 43, 45–50, 53, 54, 56, 58]
Joint production	The performing of multiple tasks at the same time	3 [42, 58, 60]
Time-bound tasks	The degree to which certain caregiving tasks have to be performed at the same time each day	1 [42]
Time measurement method	Different means of collecting informal time care information (e.g. diary vs recall methods)	6 [28, 39, 41, 57, 58, 60]
Intangible tasks	Tasks that have no tangible end product such as social and emotional care	4 [22, 44, 59, 60]
Carer and recipient identification	Informal care information compared between carers or recipients within the same dyad, or from one perspective across different measurement methods	2 [39, 55]
Multiple caregivers	Informal care provided by someone who is not the primary caregiver	2 [50, 51]
Aggregation of informal care tasks	Means of collecting informal care information through list-based tasks or one aggregated question	1 [58]
Non-response bias	Carers may be more or less likely to take part in surveys (unit non-response) or answer caregiving questions (item non-response) than non-carers	1 [52]