A structure factor sums signed scattering contributions before squaring for intensity, so adding amplitudes first matters even when each term has unit magnitude. In this sign-only model two same-phase terms give F = 1 + 1 = 2, so the intensity is 2 squared = 4 exactly for the stated toy scattering ledger; real structure factors are complex sums with atomic form factors, background subtraction, and calibration uncertainty.
A simplified peak model adds signed scattering contributions before squaring. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Signed contributions add first
In this sign-only training model, two same-phase terms add to 2.
F=1+1=2
Intensity follows the squared sum
The training intensity is 2 squared, so the exact result is 4. Real structure factors are complex sums with atomic form factors.