  A common assumption in analyses of error thresholds and quantum computing in
general is that one applies fault-tolerant quantum error correction (FTQEC)
after every gate. This, however, is known not to always be optimal if the FTQEC
procedure itself can introduce errors. We investigate the effect of varying the
number of logical gates between FTQEC operations, and in particular the case
where failure of a postselection condition in FTQEC may cause FTQEC to be
skipped with high probability.
  By using a simplified model of errors induced in FTQEC, we derive an
expression for the logical error rate as a function of error-correction
frequency, and show that in this model the optimal frequency is relatively
insensitive to postselection failure probability for a large range of such
probabilities. We compare the model to data derived from Monte Carlo simulation
for the $[[7,1,3]]$ Steane code.
