1. Searching for trials: Inclusion and exclusion criteria for eligibility.

General aspects
Accepted study designs	Studies with contemporary control groups (RCT or observational)
Accepted date of publication	1995‐2011
Accepted publication type	RCTs: full text, abstract, conference presentation Observational studies: full‐text only
Inclusion criteria
I1	Patients with previously untreated symptomatic MM (excluding the prognostically very different plasma cell leukaemia (International Myeloma Workshop 2003)) comprising at least 85% of the study population
I2	Intervention group: tandem autologous stem cell transplantation
I3	Control group: single autologous stem cell transplantation
I4	At least one of patient‐relevant outcomes reported (OS, EFS, transplantation‐ or treatment‐related mortality)
I5	Observational studies: comparability of study groups with detailed information for at least 5 relevant prognostic criteria per arm
Exclusion criteria
E1	Other research questions (e.g. basic research, other therapeutic approaches, other diseases, prognostic studies) or publication type (e.g. narrative review, editorial, letter without original data)
E2	Duplicate publication without additional information1
E3	Report without quantifiable/assignable outcome measures of first‐line treatment in patients with MM
E4	No acceptable study design (or, in addition, publication type for observational studies)
E5	Low number of evaluable patients per arm (< 25 patients per arm for RCTs, < 50 for observational studies)2
E6	Other Language of publication than English, French or German

1 For cumulative reports such as registry data, only the last publication was included. Abstract presentations of included RCTs published in full‐text were only included if they contained additional data (e.g. long‐term follow‐up).

2 In view of the lower relevance of non‐randomised studies for assessment of intervention effects, larger studies were required for inclusion.