  The theory of Type~I migration has been widely used in many studies.
Transiting multi-planet systems offer us the opportunity to examine the
consistency between observation and theory, especially for those systems
harbouring planets in Mean Motion Resonance (MMR). The displacement these
resonant pairs show from exact commensurability provides us with information on
their migration and eccentricity-damping histories. Here, we adopt a
probabilistic approach, characterized by two distributions -- appropriate for
either the resonant or non-resonant planets -- to fit the observed planet
period ratio distribution. With the Markov chain Monte Carlo (MCMC) method, we
find that ${\approx}15\%$ of exoplanets are in first order ($j+1{:}j$) MMRs,
the ratio of eccentricity-to-semi-major axis damping is too high to allow
overstable librations and that the results are by-and-large consistent with
Type-I migration theory. In addition, our modeling finds that a small fraction
of resonant pairs is captured into resonance during migration, implying late
planet formation (gas-poor). Most of the resonant pairs park themselves at the
migration barrier, indicating early planet formation (gas-rich). Furthermore,
after improving the criterion on two-body resonant trapping, we obtain an upper
limit of the disc surface density at the time the planets are locked in
resonance.
