  We present an analysis of 507 spectra of 173 stripped-envelope (SE)
supernovae (SNe) discovered by the untargeted Palomar Transient Factory (PTF)
and intermediate PTF (iPTF) surveys. Our sample contains 55 Type IIb SNe (SNe
IIb), 45 Type Ib SNe (SNe Ib), 56 Type Ic SNe (SNe Ic), and 17 Type Ib/c SNe
(SNe Ib/c). We compare the SE SN subtypes via measurements of the
pseudo-equivalent widths (pEWs) and velocities of the He I $\lambda\lambda5876,
7065$ and O I $\lambda7774$ absorption lines. Consistent with previous work, we
find that SNe Ic show higher pEWs and velocities in O I $\lambda7774$ compared
to SNe IIb and Ib. The pEWs of the He I $\lambda\lambda5876, 7065$ lines are
similar in SNe Ib and IIb after maximum light. The He I $\lambda\lambda5876,
7065$ velocities at maximum light are higher in SNe Ib compared to SNe IIb. We
have identified an anticorrelation between the He I $\lambda7065$ pEW and O I
$\lambda7774$ velocity among SNe IIb and Ib. This can be interpreted as a
continuum in the amount of He present at the time of explosion. It has been
suggested that SNe Ib and Ic have similar amounts of He, and that lower mixing
could be responsible for hiding He in SNe Ic. However, our data contradict this
mixing hypothesis. The observed difference in the expansion rate of the ejecta
around maximum light of SNe Ic
($V_{\mathrm{m}}=\sqrt{2E_{\mathrm{k}}/M_{\mathrm{ej}}}\approx15,000$ km
s$^{-1}$) and SNe Ib ($V_{\mathrm{m}}\approx9000$ km s$^{-1}$) would imply an
average He mass difference of $\sim1.4$ $M_{\odot}$, if the other explosion
parameters are assumed to be unchanged between the SE SN subtypes. We conclude
that SNe Ic do not hide He but lose He due to envelope stripping.
