Variables x and y are related by the equation y = p – qx where p and q are constants.

Question 2

Variables x and y are related by the equation y = p – qx where p and q are constants.

Values of x and y are measured experimentally. The results contain a systematic error.

Which graph best represents these results?

Reference: Past Exam Paper – November 2012 Paper 12 Q6

Solution:

Answer: A.

Equation: y = p – qx  

Comparing with the equation of a line y = mx + c, the gradient is negative and constant (m = -q). So, it is a straight line. [C and D incorrect]

Systematic error is due to the instrument, so all points should lie on the line as the error always occurs in the same direction.

From the equation, when x = 0, y = p, but due to systematic error, this value is reduced.

Choice A is correct as it shows no random error but a systematic error with the line not going through p.

Choice B shows random error without systematic error.