Consistency and reliability of smoking-related variables: longitudinal study design in asthma and COPD

Figures & data

Table 1. Comparison of asthma and COPD patients’ characteristics and smoking status according to the first postal questionnaire.

	Asthma N = 1329	COPD N = 959	ALL N = 2288	p-value
Male gender, N (%)	347 (26.1)	584 (60.9)	931 (40.1)
Mean age ± SD	54.9 ± 12.5	64.4 ± 7.0	58.9 ± 11.6,	<0.001
Smoking status^a, N (%)				<0.001
Current smoker	134 (10.1)	346 (36.1)	480 (19.2)
Former smoker	427 (32.1)	453 (47.2)	880 (38.5)
Never smoker	672 (50.6)	32 (3.3)	704 (30.8)
Did not answer	21 (1.6)	26 (2.7)	47 (2.1)
Questionnaire was not returned	70 (5.3)	72 (7.5)	142 (7.7)
Deaths	5 (0.4)	30 (3.1)	35 (1.5)
Mean pack years^a	15.1 ± 17.1	43.5 ± 24.3	32.3 ± 25.9	<0.001
Cigarettes smoked per day – current smokers^a, N (%)				<0.001
<10	43/134 (32.1)	40/346 (11.6)	83/480 (17.3)
10–19	54/134 (40.3)	124/346 (35.9)	178/480 (37.1)
20–29	28/134 (20.9)	132/346 (38.2)	160/480 (33.3)
>30	5/134 (3.7)	46/346 (13.3)	51/480 (10.6)
Missing data	4/134 (3.0)	4/346 (1.2)	8/480 (1.7)
Cigarettes smoked per day – former smokers^a, N (%)				<0.001
<10	123/427 (28.8)	21/453 (4.6)	144/880 (16.4)
10–19	127/427 (29.7)	75/453 (16.6)	202/880 (23.0)
20–29	105/427 (24.6)	202/453 (44.6)	307/880 (34.9)
>30	22/427 (5.2)	107/453 (23.6)	129/880 (14.7)
Missing data	50/427 (11.7)	48/453 (10.6)	98/880 (11.1)

^adata based on first follow up (one year after enrolment).

Table 2. Comparison of smoking status between the disease groups throughout the 10-year follow-up.

	Asthma N = 1154^a	COPD N = 69 8^a	All N = 1852^a	p-value
Stable, N (%)	985 (85.4)	469 (67.2)	1454 (78.5)	<0.001
Smoking (%)	63 (6.4)	122 (26.0)	185 (12.7)
Non-smoking (%)	922 (93.6)	347 (74.0)	1269 (87.3)
Unstable, changing once, N (%)	60 (5.2)	150 (21.5)	210 (11.3)	<0.001
Smoking^b	10 (16.7)	32 (21.3)	42 (20.0)
Non-smoking^b	50 (83.3)	118 (78.7)	168 (80.0)
Unstable, changing more than once, N (%)	42 (3.6)	63 (9.0)	105 (5.7)	<0.001
Smoking^b	23 (54.8)	33 (52.4)	56 (53.3)
Non-smoking^b	19 (45.2)	30 (47.6)	49 (46.7)
Unreliable, N (%)	67 (5.8)	16 (2.3)	83 (4.5)	0.0004

^aincluding all patients who had answered at least three times to the smoking-related questions during the 10-year follow-up

^bin their last report

Table 3. Variation of self-reported starting year, stopping year and consumption of cigarettes.

		Difference in responses
	N	Mean SD^a	Median SD^a	ICC (95% CI)^b
Starting Year	1010	2.51	1.20	0.85 (0.84–0.87)
Stable	724	2.09	1.08	0.87 (0.86–0.89)
Unstable 1	169	3.40	1.79	0.78 (0.74–0.82)
Unstable 2	88	4.11	1.97	0.82 (0.77–0.87)
Unreliable	29	2.99	1.70	0.91 (0.85–0.95)
Stopping year	782	2.01	0.89	0.94 (0.94–0.95)
Stable	635	1.92	0.84	0.94 (0.93–0.94)
Unstable 1	84	2.44	0.58	0.88 (0.85–0.92)
Unstable 2	37	2.69	2.08	0.88 (0.82–0.93)
Unreliable	26	1.94	1.08	0.98 (0.97–0.99)
Consumption of Cigarettes	1084	3.74	2.36	0.74 (0.73–0.76)
Stable	774	3.57	2.36	0.77 (0.75–0.79)
Unstable 1	182	4.35	3.39	0.57 (0.51–0.64)
Unstable 2	96	4.09	3.42	0.70 (0.60–0.81)
Unreliable	32	3.10	1.44	0.71 (0.64–0.78)

^aSD = Standard deviation

^bICC = Intraclass coefficient correlation, counted for patients who answered at least three times to the certain question. CI = Confidence interval

Figure 1. Variation of self-reported starting (left) and stopping (right) years within the four groups, into which the patients were divided according to their smoking statuses throughout the study.