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

Table 12 Meta-analyses for age started to smoke^a

Age started	Statistic^b	COPD	CB	Emphysema
Number of sets^c		10	2	2
About age 26 years^{d, g}	N	6	Insufficient data	Insufficient data
	F	1.74 (1.29-2.34)
	R	1.91 (1.25-1.91)
	H, P_H	1.48, NS
About age 18 years^{e, g}	N	6	Insufficient data	Insufficient data
	F	1.96 (1.60-2.41)
	R	2.11 (1.08-4.11)
	H, P_H	7.43, < 0.001
About age 14 years^{f, g}	N	8	Insufficient data	Insufficient data
	F	3.34 (2.74-2.08)
	R	3.12 (2.07-4.70)
	H, P_H	2.88, < 0.01
Earliest v latest	N	14	6	2
	F	1.41 (1.30-1.52)	1.99 (1.42-2.79)	1.14 (0.70-1.88)
	R	1.49 (1.26-1.76)	2.08 (1.29-3.35)	1.14 (0.70-1.88)
	H, P_H	1.29, NS	1.54, NS	0.01, NS

^a Within each study, results are selected in the following order of preference, within each sex, for: smoking status-current, ever; unexposed group (where relevant)-never any product, never cigarettes; 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. Results adjusted for the most potential confounders are selected.
^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 never smoked.
^d Category for which results are provided includes 26 years but does not include 18 years.
^e Category for which results are provided includes 18 years but does not include 14 or 26 years.
^f Category for which results are provided includes 14 years but does not include 18 years.
^g Base for comparison is never smoked

ISSN: 1471-2466