Table 1

Selected characteristics of studies included in the systematic review and meta-analysis

Case control studies
First author, year of publication	Location	Participants (cases/controls)	Period of recruitment	Type of cases	Controls	Case ascertainment	Diagnostic criteria	Response rate (%) cases/controls	Age (years) (mean (range))	% Men	Adjustment variables
Pierce-Ruhland, 19819	Houston, TX, USA	80/78	NS	Prevalent	Friends	Neurology clinic	Ad hoc	91/NS	52 (NS)	66	NS
Kondo-I, 198110	Japan	712/367	1965–1966	Deaths	Spouses	Mortality registry	NA	76/NS	NS	NS	NS
Kondo-II, 198110	Japan	158/158	1973	Prevalent	Community/hospital	Neurology clinic	Ad hoc	NS	NS	66	NS
Granieri, 19883	Ferrara, Italy	72/212	1982	Prevalent	Hospital	Neurology clinic	Ad hoc	NS	NS	61	NS
Provinciali, 199011	Ancona, Italy	77/80	1979–1987	Prevalent	Other neurological diseases	Neurology clinic	Ad hoc	NS	58 (NS)	74	NS
Savettieri, 199112	Palermo, Italy	46/92	NS	Prevalent	Friends/neighbours	Neurology clinic	Ad hoc	NS	NS	54	Age, sex, region, SES
Chancellor, 19934	Scotland	103/103	1990–1991	Prevalent	Community	ALS register	Ad hoc	70/NS	65 (35–96)	59	Age, sex, region
Vinceti, 199713	Reggio Emilia, Italy	16/39	NS	Prevalent	Community	ALS clinic	EEC	NS	65 (NS)	NS	Age, sex
Kamel, 199914	Boston, MA, USA	110/256	1993–1996	Incident	Community	ALS clinic	Ad hoc	71/76	56 (30–80)	61	Age, sex, region, education
Nelson, 200015	Washington State, USA	161/321	1990–1994	Incident	Community	Multiple sources	Ad hoc	97/79	61 (NS)	55	Age, sex, education, alcohol
Qureshi, 200616	Boston, MA, USA	95/106	1998–2002	Prevalent	Friends/relatives	ALS clinic	EEC	NS	54 (NS)	63	Age, sex
Sutedja, 200717	Utrecht, The Netherlands	364/392	2001–2005	Incident	Friends	ALS clinic	EEC	76/79	60 (24–83)	63	Education, occupation, age
Schmidt, 20085 34	USA	204/410	2003–2007	Incident	US Army veterans	US army veterans ALS registry	EEC	85/30	62 (NS)	95	Age, sex, race, education
Okamoto, 200920	Tokai, Japan	153/306	2000–2005	Prevalent	Community	Neurology clinics	EEC	56/42	64 (NS)	60	Age, sex, physical activity, diet, trauma
Binazzi, 200922	Rome, Italy	77/185	2005–2006	Prevalent	Community	Neurology clinics	rEEC	NS	58 (28–84)	37	Age, sex

Cohort studies
First author, year of publication	Location	Participants (cases/cohort size)	Period of recruitment	Average follow-up	Case ascertainment	Diagnostic criteria	Retention rate	Age at baseline (mean (range))	% Men	Adjustment variables
Weisskopf, women, 200418	USA	291/638 849	1982	10 years	NDI	NA	100%	56 (NS)	0	Age, alcohol, education
Weisskopf, men, 200418	USA	330/459 360	1982	10 years	NDI	NA	100%	56 (NS)	100	Age, alcohol, education
Fang, 200619	Sweden	160/280 558	1978–1993	19.6 years	Inpatient register	NS	100%	41 (NS)	100	Age, area of residence
Wang, 200823	USA	633/1 130 644	NS	NS	NDI/self-report	NS	NS	NS	51	Multivariate (NS)
Gallo, 200921	Europe	116/505 355	1991–2001	8.9 years	Death certificates	NA	NS	51 (NS)	42	Age, sex, education, study centre

ALS, amyotrophic lateral sclerosis; EEC, El Escorial Criteria; NA, not applicable; NDI, US National Death Index; NS, not specified; rEEC, revised El Escorial Criteria (Airlie House Criteria); SES, socioeconomic status.