Placebo-outcome test

Runs the *same* shift-share IV but on an outcome that the treatment should not move (a placebo). A coefficient far from zero signals that the design is picking up something other than the intended channel. This is distinct from [ssb_pretrend()], which regresses a *pre-period* outcome on the instrument (reduced form) to look for differential pre-trends.

Usage

ssb_placebo(design, placebo_y, methods = NULL, level = 0.95)

Arguments

design

An [ssb_design()] object.

placebo_y

Column name of the placebo outcome in `data`.

methods

Standard-error methods (see [ssb_estimate()]); `NULL` (default) uses the route-appropriate methods.

level

Confidence level.

Value

An `ssb_estimate` for the placebo outcome.

Examples

sim <- ssb_simulate(n_loc = 80, n_sec = 10, seed = 1)
sim$data$y_placebo <- stats::rnorm(nrow(sim$data))   # an unrelated outcome
d <- ssb_design(sim$data, sim$shares, sim$shocks, exogenous = "share")
ssb_placebo(d, placebo_y = "y_placebo")
#> <ssBartik placebo test>
#>   route         : exogenous SHARE -> conventional (EHW / cluster) inference
#>   first-stage F : 19.8
#>   full IV re-estimated with a placebo outcome, which should be unaffected
#>  method estimate std.error conf.low conf.high note
#>     EHW   0.0975     0.203     -0.3     0.495