Vectors questions reward clear method and exact notation. Students who understand the geometry still lose marks on notation, magnitude arithmetic and the final "show that" line. Here's how examiners split them.
AB = b − a (position of B minus position of A).The classic loss: subtracting the wrong way round (a − b). Method mark often holds, accuracy goes.
For |AB|: M1 for a correct use of √(x² + y² + z²), A1 for the exact value (surd where required). Squaring slips and rounding a surd to a decimal are the usual accuracy losses.
M1 for the dot-product method (a·b = |a||b|cosθ), A1 for the correct cosθ, A1 for the angle. Forgetting to divide by the magnitudes, or using the position vectors instead of the direction vectors, is where the method mark survives but the answer is wrong.
"Show that A, B, C are collinear" needs two things for full marks: showing one vector is a scalar multiple of another (M1/A1), and stating they share a common point (the final mark). Students prove the parallel part and forget the common-point statement — and lose the last mark on an otherwise perfect answer.
Award the M for the correct vector method, check components/magnitude for accuracy, and on "show that" look specifically for the concluding statement (scalar multiple + common point). Point the student to the missing line — that's the mark they'll otherwise keep losing.
Mark vectors — and the whole paper — point by point against your board's scheme.
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