Table 5

Heterogeneity of the oral cancer smokeless tobacco relative risk, unadjusted and adjusted for cigarette smoking (based on the estimates given in Table 3).

Unadjusted for any factor

Adjusted for smoking


Heterogeneityb

Heterogeneityb

Factor

N

OR/RR (95%CI)a

χ2

I2

p(χ2)c

N

OR/RR (95%CI)a

χ2

I2

p(χ2)c


All estimatesd

19

1.58 (1.39–1.78)

232.1

92.2

< 0.0001

15

1.31 (1.13–1.53)

28.7

51.3

0.0114

Type

Chewing tobacco

3

3.20 (2.12–4.82)

23.8

91.6

< 0.0001

4

1.29 (0.99–1.69)

5.6

46.2

0.1342

Snuff

3

27.9 (17.0–45.9)

19.6

89.8

0.0001

4

1.62 (1.24–2.11)

13.4

77.7

0.0038

Smokelesse

13

1.18 (1.03–1.35)

31.5

61.9

0.0017

7

1.09 (0.85–1.42)

5.4

0.0

0.4967

Between levels

157.3

67.7

< 0.0001

4.3

15.1

0.1142

Sex

Female

4

20.3 (12.6–32.5)

35.0

91.4

< 0.0001

2

2.51 (1.73–3.64)

3.5

71.5

0.0610

Male

7

1.27 (1.07–1.51)

60.6

90.1

< 0.0001

10

1.15 (0.97–1.37)

10.8

16.4

0.2925

Both

8

1.35 (1.11–1.64)

16.5

57.5

0.0213

3

1.19 (0.66–2.15)

0.6

0.0

0.7573

Between levels

120.1

51.7

< 0.0001

13.9

48.4

0.0010

Study design

Case-control, hospital controls

13

1.90 (1.64–2.21)

200.4

94.0

< 0.0001

7

1.41 (1.18–1.68)

22.0

73.7

0.0009

Case-control, population controls

6

1.01 (0.81–1.27)

11.0

54.6

0.0514

3

0.99 (0.69–1.42)

0.0

0.0

0.9935

Case-control, mixed controls

0

--

--

--

--

2f

1.35 (0.44–4.13)

--

--

0.1337

Cohort

0

--

--

--

--

3

1.24 (0.65–2.36)

--

--

0.7062

Between levels

20.7

8.9

< 0.0001

3.0

10.4

0.3928

Study location

USA

15

1.76 (1.54–2.02)

215.6

93.5

< 0.0001

10

1.39 (1.17–1.64)

23.7

62.1

0.0047

Scandinavia

2

0.93 (0.67–1.30)

1.1

5.3

0.3041

3

0.99 (0.68–1.45)

0.2

0.0

0.9054

Other

2g

0.99 (0.53–1.84)

1.3

24.6

0.2495

2h

1.35 (0.44–4.13)

2.2

55.5

0.1337

Between levels

14.2

6.1

0.0008

2.5

8.8

0.2810

Study period

-1969

5

4.49 (3.55–5.66)

79.8

95.0

< 0.0001

4

2.02 (1.28–3.20)

3.6

15.8

0.3129

1970–1979

5

1.05 (0.84–1.33)

33.0

87.9

< 0.0001

1

2.67 (1.83–3.90)

0.0

0.0

--

1980–1989

7

1.01 (0.83–1.23)

10.4

42.4

0.1080

2

0.97 (0.71–1.31)

0.0

0.0

0.8743

1990+

2

1.37 (0.64–2.96)

0.0

0.0

0.8321

8

1.10 (0.88–1.37)

1.8

0.0

0.9719

Between levels

108.9

46.9

< 0.0001

23.4

81.4

< 0.0001


a Fixed-effect estimates

b For the "Between levels" rows, the I2 column contains the percentage of variation explained by the factor levels rather than the heterogeneity I2 statistic

c The probability for the heterogeneity χ2 between studies within levels is shown, except that for the "Between levels" rows, the probability is for the between levels χ2

d For each study/sex, the OR/RR for smokeless tobacco from Table 3 was included if available, otherwise that for chewing tobacco or snuff was used. For study 25 where estimates for snuff and chewing tobacco, but not smokeless tobacco, were available, that for chewing tobacco was included as it was for smokers and nonsmokers combined

e This includes all the OR/RR estimates from Table 3 where the exposure was smokeless, both from studies where only estimates for unspecified exposure were available and from studies where it had been estimated from the separate data for chewing tobacco and snuff

f In study 7 (two estimates), both hospital and population controls were used

g Studies 9 (UK) and 18 (Brazil)

h Study 7 (conducted in Puerto Rico, two estimates)

I Since only one study with an effect estimate adjusted for smoking was conducted between 1970 and 1979 (study 13), an additional assessment of heterogeneity was carried out based on two periods (-1979 vs. 1980+). The corresponding overall effect estimates, based on 5 and 10 studies respectively, were 2.39 (1.78–3.19) and 1.05 (0.88–1.26), with p < 0.0001 for between levels heterogeneity

Weitkunat et al. BMC Public Health 2007 7:334   doi:10.1186/1471-2458-7-334

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