Particle size analysers using the electrical-zone method assume that particles pass through an orifice tube one at a time. In fact, two or more particles will sometimes pass through the orifice tube roughly simultaneously. This is called coincidence, and is a source of errors in the measurements from these devices. Coincidence reduces the observed number of particles and increases the observed average particle size. The first effect can be corrected simply, but the correction of the second effect is much more difficult. In the present work, a simple model of coincidence is developed, including the effects of coincidence on the observed particle size distribution. A numerical method is presented that reverses these effects; the method requires a novel technique of numerical Fourier transform. Experimental results are presented, and the correction procedure is applied. The success of this correction is assessed. Some errors remain : most notably, some of the total particle volume is lost when coincidence is prevalent. This error probably originates in the data-processing performed by the measuring device. In general, it is found that the procedure improves the measurements even when the level of coincidence is very high.