[-kit-kat-]

ok, I'm doing S1 edexcel this summer and In doing a past paper at the moment, but I dont understand about skewness

My books have hardly anything about it. I get that for a graph, data is positively skewed if the high bit is on the left, and vice versa for negative.
However I've been doing a question and it gave a formula for "the coefficient of skewness" so i plugged in the values and got skewness=0.139. It then says comment on this. Am I right in saying that the value I got is positive, so the skewness is possitive? or is that not right?

[freeze99]

(Original post by -kit-kat-)
ok, I'm doing S1 edexcel this summer and In doing a past paper at the moment, but I dont understand about skewness

My books have hardly anything about it. I get that for a graph, data is positively skewed if the high bit is on the left, and vice versa for negative.
However I've been doing a question and it gave a formula for "the coefficient of skewness" so i plugged in the values and got skewness=0.139. It then says comment on this. Am I right in saying that the value I got is positive, so the skewness is possitive? or is that not right?

yh just say positively/negatively skewed

If Q3-Q2 > Q2-Q1 - Positive Skew
If Q3-Q2 < Q2-Q1 - Negative Skew
If Q3-Q2 = Q2-Q1 - No Skew / Symmetrical


Hope that helps

[-kit-kat-]

(Original post by freeze99)
yh just say positively/negatively skewed

If Q3-Q2 > Q2-Q1 - Positive Skew
If Q3-Q2 < Q2-Q1 - Negative Skew
If Q3-Q2 = Q2-Q1 - No Skew / Symmetrical


Hope that helps

thanks
The question gave a formula for the coefficient of skewness - what is that?

[freeze99]

(Original post by -kit-kat-)
thanks
The question gave a formula for the coefficient of skewness - what is that?

I think its just another way of finding the skewness for it.

[-kit-kat-]

(Original post by freeze99)
I think its just another way of finding the skewness for it.

but how do you use it to do that?

[abbii]

These are my skewness notes for S1 which explain the coefficient of skewness at the bottom.

In a symmetrical distribution mean = median = mode and Q2 – Q1 = Q3 – Q2. A positive skew distribution has a tail to the positive side, mean > median > mode and Q2 – Q1 < Q3 – Q2. A negative skew distribution has a tail to the negative side, mean < median < mode and Q2 – Q1 > Q3 – Q2. Skewness can be given quantity as well as direction by using (3 (mean-median))/(standard deviation) . To compare the relative dispersion between data sets the coefficient of variation is used, it is given by V= 100σ/μ= 100s/x ̅ and as a percentage. When data is skewed the median and the interquartile range are used as measures of location and dispersion as they are not affected by extreme values. The dispersion of a set of data is therefore measured by the quartile coefficient of variation which is given by QV= (50 (Q_3- Q_1))/Q_2 .

x ̅ is mean of x and Q_3 is the upper quartile mark, if the notation wasnt that clear

[.ACS.]

http://en.wikipedia.org/wiki/Skewness

Although it's never good to use Wikipedia, the article isn't too bad for what's needed at A-Level.

The most common formula for skewness is Pearson's first, which is:
Though, I think Edexcel use this one:

Although far beyond A-Level, this is quite an interesting read:
http://www.amstat.org/publications/j...vonhippel.html