Table 2.

Patterning of cognitive scores over time in early childhood, ECLS-B unweighted study sample (N = 7,200)

	Number Not	Number	Percent (95% CI)
	Delayed at 48 Mo	Delayed at 48 Mo	Delayed at 48 Mo
I. All children cognitively delayed at 24 Mo (n = 1,000)	750	250	24.2a (21.6, 27.0)
II. All children not cognitively delayed at 24 Mo (n = 6,250)	5,750	500	7.9 (7.2, 8.6)

The Bayley mental score was used as the cognitive score at 24 months; at 48 months, literacy, math, color knowledge, and receptive vocabulary scores were converted into z-scores and summed. At both 24 and 48 months, cognitive delay was defined as falling in the lowest 10% of scores

Cutoffs for the bottom 10% of scores at 24 and 48 months were determined from the weighted, full available sample of cases at each time point. (This, plus oversampling of low birth weight and multiple birth children accounts for the fact that the analysis sample contains more than 10% delayed children at 24 months.)

All sample sizes in this table are rounded to the nearest 50 in accordance with ECLS-B data confidentiality requirements (available at http://nces.ed.gov/ecls/birthdatainformation.asp)

^aPercentages and confidence intervals are calculated from actual sample sizes and have not been rounded