Table 4.

Practice variation in average record lengths in the DIN and GPRD birth cohorts

	DIN (n = 141)				GPRD (n = 464)
	Lower quartile	Median	Upper quartile	% variation between practices due to sampling variation	Lower quartile	Median	Upper quartile	% variation between practices due to sampling variation
No. of days with a prescription issued	16.4	18.5	22.3	5.2%	15.5	18.4	22.5	8.3%
No. of days with diagnostic codes	10.6	15.4	20.0	1.5%	18.5	21.9	25.6	8.5%
No. of days with diagnostic codes or drugs linked to existing diagnostic codes	18.8	22.9	27.3	4.8%	_	_	_	_
No. of days with either diagnostic or drugs or both codes	21.1	25.6	30.8	4.8%	22.5	25.7	30.7	9.9%

– the 25^th, 50^th and 75^th percentiles of the practice mean number of events in the first 5 years of life – see statistical methods