Table 2.

NDSS Symptom Item Response Parameter Estimates

Symptom	Nondaily Smokers		Daily Smokers		Δχ2 p value	Effect Size for Severity DIF
	Discrimination (slope)	Severity (threshold)	Discrimination (slope)	Severity (threshold)
Drive
After not smoking for a while, you need to smoke to feel less restless and irritable	1.20	1.05	0.84	0.97	0.009*	0.08
When you don’t smoke for a few hours, you start to crave cigarettes*	1.39	1.42	0.96	1.36	0.000*	0.06
You sometimes have strong cravings for a cigarette where it feels like you are in the grip of a force that you can’t control	0.94	1.68	0.94	1.68	0.766	NA
You feel a sense of control over your smoking – that is you can “take it or leave it” at any timea	0.54	0.31	0.54	0.31	0.067	NA
You sometimes worry that you will run out of cigarettes	1.44	1.87	0.87	1.98	0.000*	0.11
Priority
You tend to avoid places that don't allow smoking, even if you would otherwise enjoy them	0.32	2.71	0.32	2.71	0.560	NA
Even if you’re travelling a long distance, you’d rather not to travel by plane because you wouldn’t be allowed to smoke	0.73	3.05	0.73	3.05	0.447	NA
Continuity
It’s normal for you to smoke several cigarettes in an hour, then not have another one until hours latera	0.81	1.52	0.46	1.42	0.000*	0.10
You smoke cigarettes fairly regularly throughout the day	1.72	1.41	0.73	0.79	0.000*	0.62
You smoke about the same amount on weekends as on weekdays	0.95	0.96	0.21	−0.80	0.000*	1.76
You smoke just about the same number of cigarettes from day to day	1.21	0.97	0.42	−1.50	0.000*	2.47
It’s hard to say how many cigarettes you smoke per day because the number often changesa	0.88	0.55	0.32	0.63	0.000*	0.08
Tolerance
Since you started smoking, the amount you smoke has increased	1.40	0.84	1.40	0.84	0.080	NA
Compared to when you first started smoking, you need to smoke a lot more now in order to be satisfied	1.77	1.46	1.77	1.46	Anchor	NA
Compared to when you first started smoking, you can smoke much, much more now before you start to feel anything	1.17	1.12	1.17	1.12	0.702	NA
Stereotypy
Your smoking is not affected much by other things. For example, you smoke the same amount whether you’re relaxing or working, happy or sad, alone or with others	0.50	0.38	0.09	−3.84	0.000*	4.22
The number cigarettes you smoke per day is often influenced by other things – how you’re doing, what you’re feeling, for examplea	0.89	0.14	0.89	0.14	0.471	NA

Note: The Δχ² p value represents the p value for the change in chi square between the model in which the discrimination and severity parameters were free to vary across groups (baseline model) and the model in which these parameters for the symptom of interest were constrained to be equal across groups.

^areverse coded.

^*significant using the Benjamini-Hochberg procedure for Type I error rate adjustment for multiple tests (Thissen et al., 2002)