  The nature of quasi-periodic pulsations in solar flares is poorly
constrained, and critically the general prevalence of such signals in solar
flares is unknown. Therefore, we perform a large-scale search for evidence of
signals consistent with quasi-periodic pulsations in solar flares, focusing on
the 1 - 300s timescale. We analyse 675 M- and X-class flares observed by GOES
in 1-8\AA\ soft X-rays between 2011 February 1 and 2015 December 31.
Additionally, over the same era we analyse Fermi/GBM 15-25 keV X-ray data for
each of these flares that was associated with a Fermi/GBM solar flare trigger,
a total of 261 events. Using a model comparison method, we determine whether
there is evidence for a substantial enhancement in the Fourier power spectrum
that may be consistent with a QPP signature, based on three tested models; a
power-law plus a constant, a broken power-law plus constant, and a
power-law-plus-constant with an additional QPP signature component. From this,
we determine that ~30% of GOES events and ~8% of Fermi/GBM events show strong
signatures consistent with classical interpretations of QPP. For the remaining
events either two or more tested models cannot be strongly distinguished from
each other, or the events are well-described by single power-law or broken
power-law Fourier power spectra. For both instruments, a preferred
characteristic timescale of ~5-30 s was found in the QPP-like events, with no
dependence on flare magnitude in either GOES or GBM data. We also show that
individual events in the sample show similar characteristic timescales in both
GBM and GOES datasets. We discuss the implications of these results for our
understanding of solar flares and possible QPP mechanisms.
