Inflation models based on scalar-tensor theories of gravity are formulated in
the Jordan frame but most often analysed in the Einstein frame. The
transformation between the frames is not always desirable. In this work we
formulate slow-roll conditions in the Jordan frame. This is achieved by
comparing different background quantities and approximations on the same
spatial slice in both frames. We use these approximations to derive simple
equations that can be applied to compute inflation model observables in terms
of Jordan frame quantities only. Finally, we apply some of the results to
analyse generalised induced gravity models and compare them with the latest
observations.
