I've taken on a tutee doing AS S1 (edexcel). His marked homework is always pointing out 'rounding errors' and he's very confused as he thinks he's rounding his answers correctly, so do I! However there is another problem to do with which numbers should be used. On the exam papers it says 'Values from the statistical tables should be quoted in full. When a calculator is used, the answer should be given to an appropriate degree of accuracy.' The problem is I'm unclear what this 'degree of accuracy' should be. As an example I'll go through my confusion based on May 2006 S1 paper, question 3, looking at their answers and comments. The question gives a table of data consisting of figures to 2 decimal places. a) Calculate Sxy and Sxx.(Given sum x^2= 15965.01 and sum xy = 757.467) Answer : Sxy = 71.4685 (awrt 71.5) Sxx = 1760.45875 (awrt 1760) Anyone care to tell me how these answers are 'an appropriate degree of accuracy?' You have 2 decimal places provided for most of the data, and one with 3 decimal places, neither answer matches this. Is it supposed to? b) Find the form of the regression line of y on x in the form y = a + bx. (I've skipped the coding bit as I just wanted to ask about the decimal places.) Answer: b= 0.04059652 (awrt 0.0406) c = 0.324364 (awrt 0.324) y = 0.324 + 0.0406x (3 sf or better ...) Now the answer is in significant figures, rather than decimals. Why? How are the students supposed to know this? '3 sf or better' Better than what? They have managed to give an answer to 4 decimal places which surely can't be 'an appropriate degree of accuracy!' I'm missing something and I've looked through their textbooks and they don't make clear what the 'appropriate degree of accuracy' actually is. Can anyone enlighten me? This student is loosing marks for something that isn't really based on the statistics and he is being told he has 'rounding errors'. What he really has is confusion about what 'an appropriate degree of accuracy' is and so do I.