Backward Blame
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State
Backpropagation gives every knob in a stack its blame share, passed layer by layer from output toward input in one backward sweep.
Show
A buried knob answers through the numbers it fed forward: its influence on them, times their influence on the error.
Watch for
The accounting is exact — the same numbers a ruinous knob-by-knob census would produce, at a millionth the cost.