Edexcel (IAL): Statistics 1, WST01 (27th January 2016)

For 6d), I don't understand how S.D decrease while there was no change in the mean value. I must be missing something...

Standard deviation is a measure of the *spread* of the data. If you add in two points that are closer to the mean than the others are (on average), you *decrease* the spread of the data and hence the standard dev.

Imagine the mean to be one line and if all the data lies on the line (horizontal line) then the spread is 0. If the data is spread top and bottom, the spread in increases. You can add a data point that is far away from the mean and a data point that is far below the mean that keeps the mean constant because they cancel each other out but increases the standard deviation because the data is more spread.

Reply 141

WilsonDS

1

Original post by Zacken

Standard deviation is a measure of the *spread* of the data. If you add in two points that are closer to the mean than the others are (on average), you *decrease* the spread of the data and hence the standard dev.

Imagine the mean to be one line and if all the data lies on the line (horizontal line) then the spread is 0. If the data is spread top and bottom, the spread in increases. You can add a data point that is far away from the mean and a data point that is far below the mean that keeps the mean constant because they cancel each other out but increases the standard deviation because the data is more spread.

Thanks, I get it now.
There goes my mark then *sigh*

Reply 142

12103763d

10

In Q2, last part, I assumed it is a given condition and without replacement. Thus, my ans was 0.25*0.24*0.5*3 / 0.75*0.74*0.73, it sounds weird lol, lost 3 marks anyway~