An apparatus is provided comprising receiving circuitry to receive a representation of a circuit comprising a plurality of flops. Categorisation circuitry determines data dependencies between the flops from the representation and generates a categorisation of the flops into one of at least: a vulnerable category, a conditional category, and an isolated category, in dependence on the data dependencies. The categorisation indicates the vulnerability of the flops to transient errors. Output circuitry outputs the categorisation of the flops. The conditional category comprises those of the flops whose change in value is indicated by a change in a value in a corresponding flop in the flops or corresponding signal. The vulnerable category comprises those of the flops that are absent from the conditional category and whose change in value is affected by one of the flops or a signal and the isolated category comprises the flops that are absent from the conditional category and that are absent from the vulnerable category.