Reduced-form regression of a pre-period outcome on the constructed instrument (controls partialled out). A coefficient far from zero indicates that exposure predicts differential pre-trends — a threat to identification. This is distinct from [ssb_placebo()], which runs the *full IV* on a placebo outcome; pre-trends ask whether exposure predicts the outcome *before* the shocks, placebo asks whether the design moves an outcome it should not.
Value
A list (class `ssb_pretrend`) with the reduced-form coefficient, EHW / cluster / exposure-robust (AKM) standard errors, the corresponding p-values (`p_ehw`, `p_akm`), and intervals (`conf.low`/`conf.high` use the EHW SE; `conf.low_akm`/`conf.high_akm` the exposure-robust SE).
Details
Because the regressor is itself a shift-share variable, EHW / cluster standard errors are subject to exactly the over-rejection documented by Adao, Kolesar & Morales (2019): residuals are correlated across units with similar exposure. The test therefore also reports an exposure-robust (AKM-type) standard error that clusters the score at the shock level; treat that one as the headline, especially on the shift route, or spurious "pre-trends" will appear too often.