Systematic review with meta-analysis of the epidemiological evidence relating smoking to COPD, chronic bronchitis and emphysema

Table 16 Meta-analyses for duration of quitting (vs. current smoking)^a

Duration of quitting	Statistic^b	COPD	CB
Number of sets^c		9	11
About 3 years^{d, g}	n	2	7
	F	0.77 (0.51-1.15)	1.07 (0.91-1.25)
	R	0.77 (0.51-1.15)	1.00 (0.70-1.43)
	H, P_H	0.28, NS	3.85, < 0.001
About 7 years^{e, g}	n	2	5
	F	1.03 (0.62-1.70)	1.00 (0.84-1.19)
	R	1.03 (0.62-1.70)	0.87 (0.47-1.61)
	H, P_H	0.53, NS	9.89, < 0.001
About 12 years^{e, g}	n	9	9
	F	0.52 (0.43-0.63)	0.61 (0.53-0.69)
	R	0.52 (0.37-0.71)	0.65 (0.41-1.04)
	H, P_H	1.93, < 0.1	9.11, < 0.001
Longest v shortest	n	10	11
	F	0.43 (0.34-0.53)	0.78 (0.64-0.95)
	R	0.45 (0.24-0.84)	0.80 (0.63-1.02)
	H, P_H	5.78, < 0.001	1.28, NS

^a Within each study, results are selected in the following order of preference, within each sex, for: smoking product-cigarettes (ignoring other products), cigarettes only, any product; overlapping studies-principal, subsidiary; and then for single sex results in preference to combined sex results.
^b n = number of estimates combined, F = fixed-effect meta-analysis RR (95% CI), R = random-effects meta-analysis RR (95% CI), H = heterogeneity chisquared per degree of freedom, P_H = probability value for heterogeneity expressed as p < 0.001, p < 0.05, p < 0.1 or NS (p ≥ 0.1).
^c Number of sets of RRs available for the key value analysis, where base for comparison is current smoking. No data available for emphysema.
^d Category for which results are provided includes quit 3 years ago but does not include quit 7 years ago.
^e Category for which results are provided includes quit 7 years ago but does not include quit 3 or 12 years ago.
^f Category for which results are provided includes quit 12 years ago but does not include quit 7 years ago.
^g Base for comparison is current smoking.

ISSN: 1471-2466