Edexcel S1 - 15th June, 2016 [Exam Discussion]

Guys, in the normal distribution questions, how are we supposed to know exactly what value of z to take when given probabilities?

For example, say I was given that the probability of getting less than a certain value X is 0.95. So I go to the table to find out what value of z will give a probability of 0.95, but the closest z values you can get are 1.64 and 1.65, which give probabilities of 0.9495 and 0.9505, respectively. I had no clue how I was supposed to choose between the two, so I just went with 1.65.

In the mark scheme, however, it specifically says that if I use 1.65, I lose a mark. This caused me to then lose another mark in the next part of the question.

So is there any way to know which one I should choose? In situations like this, do I just go with the lower z value?

You would have to choose 1.64 because 0.9495 rounds up to 0.950 (3dp) whereas if you round up 1.65 it goes to 0.951(3dp) so its better to look at it like that and choose 1.64 as the answer to use.

That makes sense. So if I wanted a value of z, I would choose whichever gave the closest probability to the one I wanted. And if they're equal, like 1.64 and 1.65, just round it. Thanks

Can someone explain why sometimes you used n+1/2 and sometimes n/2 for the median. Which one do I use and where.

Anyone have tips on how to not make stupid mistakes. I am scared ill make a stupid mistake part a and carry it through to part f. ;(

I need to get at least 65 to make sure I have a solid A. I got 60/75 for C1 and 70/75 for C2

does anyone know if we have to clear the memory from our graphical calculators before every exam or is just once enough for the whole exam period?

does anyone know if we have to clear the memory from our graphical calculators before every exam or is just once enough for the whole exam period?

Graphical calculators are banned for this exam?

Can someone explain why sometimes you used n+1/2 and sometimes n/2 for the median. Which one do I use and where.

Anyone have tips on how to not make stupid mistakes. I am scared ill make a stupid mistake part a and carry it through to part f. ;(

I need to get at least 65 to make sure I have a solid A. I got 60/75 for C1 and 70/75 for C2

depends on the type of data, whether it is discrete etc. however in mark schemes you get the marks if you do it either way

Exam solutions says do n+1/2 all the time.

Anyone know where I can get hold of good worked examples and questions for practicing Conditional Probabilities? e.g. P(AnB), P(AuB). This is the area that I seem to struggle with the most especially when trying to figure out what region of a venn diagram needs to be shaded for something more difficult e.g. P(AuB'.