I am going over Stef Van Burren's "Flexible Imputation of Missing Data" and don't understand why Table 1.1 from section 1.3 below gives the standard error for listwise deletion as too large.
In more detail, section 1.3.1 states "Under MCAR, listwise deletion produces standard errors and significance levels that are correct for the reduced subset of data, but that are often larger relative to all available data"
Why does listwise deletion produce standard errors that are correct for the reduced subset off data, but often larger relative to all available data? I don't see why the standard error wouldn't be the same under MCAR, as the missingness distribution is the same for the missing and observed data. Thank you!