Table 2. Demographic Characteristics of the Participant Sample Over Time.

Demographic category	Demographic	Sample number (chronological)
		1–100	101–200	201–300	301–398
General	Age (mean years±SEM)	23.9±0.41	22.7±0.39	23.7±0.32	22.8±0.33
General	Gender (% male)	51	61	63	34
General	Education level—% high school or some college	54	46	26	46
General	Education level—% college degree	39	39	64	47
General	Education level—% advanced	7	15	10	7
General	BMI (mean+SEM)	22.6±0.23	22.8±0.22	22.4±0.21	22.5±0.21
Ancestry	% American Indian	1	0	0	0
Ancestry	% African American	20	0	0	0
Ancestry	% Asian	12	0	0	0
Ancestry	% Caucasian	54	95	97	95
Ancestry	% Hispanic	5	5	3	5
Ancestry	% More than one race	6	0	0	0
Ancestry	% Missing	2	0	0	0
Current drug use	Alcohol (mean drinks per week±SEM)	4.22±0.34	4.49±0.38	5.89±0.54	5.68±0.47
Current drug use	Cigarettes (mean cigs per week±SEM)	0.98±0.24	0.53±0.13	1.20±0.28	1.05±0.26
Current drug use	Caffeine (mean cups per week±SEM)	9.19±0.93	5.98±0.53	8.31±0.62	8.17±0.55
Current drug use	Marijuana (mean times per month±SEM)	0.97±0.27	0.85±0.18	1.72±0.47	2.04±0.53
Lifetime substance use (ever used recreationally)	Sedatives (% yes)	5	6	9	12
Lifetime substance use (ever used recreationally)	Stimulants (% yes)	17	25	25	38
Lifetime substance use (ever used recreationally)	Opiates (% yes)	10	16	20	27
Lifetime substance use (ever used recreationally)	Hallucinogens (% yes)	37	29	32	42
Lifetime substance use (ever used recreationally)	Inhalants (% yes)	12	10	8	13
Lifetime substance use (ever used recreationally)	Marijuana (% yes)	74	74	72	86

Demographic characteristics of the sample were computed over four phases of sample collection. Mean values are expressed as mean±SEM.