How many marks does Edexcel take off for bad writing in S2?

How many marks do Edexcel take off for bad writing in S2? I’ve looked again at the recent mark schemes, and I’m not sure.

grammar

The June 2014 (R) mark scheme, for example, has in Q.3

X ~ Po(9)

Then [Y = no. of accidents in a month] Y ~ Po(1.5)

Then [A = no. of months with at least one accident] A ~ B(6, 0.777)

There’s no explanation of whether you have to include (something like) the words in square brackets to get full marks, or whether they are just there so that readers can understand the mark scheme.

The fact that they are there so that readers can understand the mark scheme means that it is good practice to include similar words in your work, so that readers can understand your work.

It is certainly good practice to have different letters (X, Y, A, whatever) for different random variables used in the same question (rather than calling them all X: like a teacher calling all students “Boy!” instead of using your individual name?).

Whether you actually lose marks for bad writing here I don’t know, and may depend on the marker and how she or he is feeling at the time.

The mark schemes are clear that in hypothesis-testing questions, you lose a mark if you just write: “There is enough evidence to reject H0” or “There is not enough evidence to reject H0”.

You lose a mark unless you also write something like:

“Not significant. There is insufficient evidence to support Thomas’ claim”.

Or: “The number/rate/amount of defects is not decreased/less/reduced”.

The second thing I’ve quoted from the Edexcel mark scheme there is wrong. It may be that the general rate of defects has in fact decreased. All we know is that so far we don’t have enough evidence to conclude it has decreased. And that is different.

Equally, if the evidence had produced a result in the critical region, all we could say is something like: “there is enough evidence to conclude that the rate of defects has decreased”, which is different from saying flat-out: “the rate of defects has decreased”.

So you can sometimes get full marks for writing things that are wrong. Better to write things that are correct, though.

In real statistics, this is not just a technicality or pedantry. In real statistics, it is a much worse error to write “the rate has not decreased”, when all you are justified in writing is “the evidence is not enough to say that the rate has decreased” (or something like that), that to get a continuity correction wrong, or something like that.

And you want to learn real statistics, not just Edexcel-statistics, no?