When can you trust reported segment margins, and when should you be skeptical?

Trust them more when: segments align with how competitors report (so you can benchmark), allocation policy is stable over time, and the margin TREND is corroborated by external evidence like peer margins and industry pricing. Be skeptical when

• corporate cost allocations are large relative to segment profit — a few points of overhead moved between segments can manufacture whatever story management wants
• intersegment sales are material, since transfer pricing sets where profit lands
• the company recently re-segmented, especially if the previously deteriorating unit conveniently disappeared into a healthier one
• one segment's margin looks implausibly stable while the business is visibly cyclical. Practical rules: rebuild history on the restated basis whenever segments change, anchor on direction rather than absolute level, and triangulate against pure-play comps serving the same end market.