  The use of pulsars as astrophysical clocks for gravitational wave experiments
demands the highest possible timing precision. Pulse times of arrival (TOAs)
are limited by stochastic processes that occur in the pulsar itself, along the
line of sight through the interstellar medium, and in the measurement process.
On timescales of seconds to hours, the TOA variance exceeds that from
template-fitting errors due to additive noise. We assess contributions to the
total variance from two additional effects: amplitude and phase jitter
intrinsic to single pulses and changes in the interstellar impulse response
from scattering. The three effects have different dependencies on time,
frequency, and pulse signal-to-noise ratio. We use data on 37 pulsars from the
North American Nanohertz Observatory for Gravitational Waves to assess the
individual contributions to the overall intraday noise budget for each pulsar.
We detect jitter in 22 pulsars and estimate the average value of RMS jitter in
our pulsars to be $\sim 1\%$ of pulse phase. We examine how jitter evolves as a
function of frequency and find evidence for evolution. Finally, we compare our
measurements with previous noise parameter estimates and discuss methods to
improve gravitational wave detection pipelines.
