to estimate the probability of this occurring. Each of the 250,000 simulated objects was sorted into one of 3,600 bins by wavelength and magnitude (each bin covering 1 Å and 0.1 magnitudes). It is then possible to calculate the completeness of the bin (i.e. the fraction of simulated objects which were recovered). Each candidate ELG is assigned to a bin, and we then check the corresponding bins in adjacent images for completeness. A low completeness value in these bins indicates that a flat-spectrum object may have been ‘lost’.

This procedure calls into question four objects: A in the field of MRC B2158-206, B in the field of MRC B2156-243 and E and K in the field of BR B0019-1522. These sources were examined by eye, but there is no indication of a faint detection in the crucial frame. They have not, therefore, been excluded from this analysis.

### Lower redshift interlopers

Another possibility is other emission lines at lower redshift may appear in our observations. The lines which might be observed are listed in Table \[tab:interlopers\].

  ------ -------- --------------- ------- -------- ------- -------- ------- -------- -- --
   Line     Å          Flux                                                             
          (rest)       ratio        $z$    Number    $z$    Number    $z$    Number     
           3727    $0.41\pm0.21$   