Pack-years | Statisticb
| COPD | CB | Emphysema |
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Number of setsc
| Â | 28 | 11 | 3 |
About 5d, g
| N | 23 | 10 | Insufficient data |
 | F | 1.13 (1.06-1.20) | 2.11 (1.74-2.55) |  |
 | R | 1.25 (1.09-1.44) | 1.74 (1.17-2.58) |  |
 | H, PH
| 2.06, < 0.01 | 2.85, < 0.01 | Â |
About 20e, g
| N | 11 | 8 | Insufficient data |
 | F | 1.68 (1.58-1.79) | 4.54 (3.69-5.58) |  |
 | R | 2.53 (1.87-3.43) | 4.54 (3.69-5.58) |  |
 | H, PH
| 4.44, < 0.001 | 0.63, NS | Â |
About 45e, g
| N | 15 | 8 | Insufficient data |
 | F | 3.14 (2.97-3.32) | 7.33 (5.98-8.97) |  |
 | R | 3.69 (2.79-4.86) | 7.04 (5.06-9.79) |  |
 | H, PH
| 6.34, < 0.001 | 1.82, < 0.1 | Â |
Highest v lowest | N | 31 | 11 | 3 |
 | F | 2.82 (2.69-2.97) | 2.52 (2.25-2.82) | 1.86 (1.40-2.47) |
 | R | 2.80 (2.37-3.30) | 3.09 (2.33-4.10) | 2.42 (1.25-4.70) |
 | H, PH
| 4.09, < 0.001 | 2.23, < 0.05 | 1.79, NS |
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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.
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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, PH = probability value for heterogeneity expressed as p < 0.001, p < 0.05, p < 0.1 or NS (p ≥ 0.1).
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c Number of sets of RRs available for the key value analysis, where base for comparison is never smoked.
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d Category for which results are provided includes 5 pack-years but does not include 20 pack-years.
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e Category for which results are provided includes 20 pack-years but does not include 5 or 45 pack-years.
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f Category for which results are provided includes 45 pack-years but does not include 20 pack-years.
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g Base for comparison is never smoked.