Why might two companies with identical pre-tax book income report very different EFFECTIVE tax rates, and how do deferred taxes fit in?

The effective rate (book tax expense ÷ book pre-tax income) is driven mainly by PERMANENT differences and discrete items, not timing. Differences arise from: tax-exempt income (munis), non-deductible expenses (fines, some goodwill impairment, certain comp), the mix of domestic vs. lower-taxed foreign earnings, R&D and other tax CREDITS, valuation-allowance changes, stock-comp windfalls/shortfalls, and tax-rate re-measurement. Temporary differences (deferred taxes) generally do NOT change the effective rate — they shift tax between current and deferred but the total book expense, and thus the ratio, is the same; they DO change the CASH tax rate. So a candidate should separate 'why is book ETR different' (permanent/discrete) from 'why is cash tax different' (deferred/timing).