I have a couple of questions about their methodology. How are they accounting for multiple comparisons in their tests for statistical significance? The answer

I have a couple of questions about their methodology. How are they accounting for multiple comparisons in their tests for statistical significance? The answer had better be really good because they are making a /lot/ of significance tests at once. How come it is reasonable to estimate the average household wealth as the median multiplied by the total number of households? This is novel to me and intuitively it seems dubious.