Correct statement about error and uncertainty

[songoku]

Homework Statement

Which of the following statements is correct?
a. Checking for zero error in voltmeter before measuring a voltage will help to reduce random error

b. Measuring the timing of a larger number of oscillations will help to reduce the fractional uncertainty of the period

c. Plotting a graph of voltage and current readings for an ohmic device and using its gradient to find resistance will help to eliminate random error

d. In the measurement of the diameter of a sphere, taking more readings and finding the average value of these readings will help to reduce the fractional uncertainty of the diameter

Relevant Equations

Not sure

I know option (a) and (c) are wrong but I don't know the difference between option (b) and (d) because I think they are the same (taking average to get more accurate reading and reduce random error).

My guess is (b) because I think for (d) the fractional uncertainty maybe can stay the same because taking average will reduce random error (the uncertainty) and also can reduce the reading of the diameter (I mean the average of 5 readings will be smaller than average of 4 readings) so the fractional uncertainty ($\frac{\Delta ~ diameter}{diameter}$) stays the same

I am not sure whether my reasoning is correct. Thanks

 

[BvU]

I mean the average of 5 readings will be smaller than average of 4 readings

No reason why it should ...
(unless the balloon is deflating )

You sure only one and not two can be correct ?

 

[songoku]

No reason why it should ...
(unless the balloon is deflating )

You sure only one and not two can be correct ?

Well I can't say I am sure but from all the questions given, usually there is only one correct answer, unless there is something wrong with the question