Electronic supplementary material
The online version of this article (doi:10.1007/s11136-012-0165-z) contains supplementary material, which is available to authorized users.
The online version of the original article can be found under doi:10.1007/s11136-011-9892-9.
The online version of the original article can be found at http://dx.doi.org/10.1007/s11136-011-9892-9.
The electronic supplementary material was missing in the original publication. Please find it published online with this erratum.
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