# Math Help - Standard Deviation help.

1. ## Standard Deviation help.

Just trying to revise with past exam papers.

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A firm recorded the number of orders recieved per weekduring 2002 to give the following frequency distribution:

i) Find the mean number of orders recieved for that time period.

ii) Find the standard deviation of the number of orders recieved

Stuck on this one

can anyone help please, i have an exam in january and i really wanna understand this.

2. How did you get a mean when the data was given in ranges? To find standard deviation, you want to subtract the mean from each and every sample point. Square that result. Now add up all of your results and divide by N - the # of total results. That is your variance. The square root of variance is standard deviation.

3. ok i understand now, I have to find the mid class values of the data first and work out the mean from there.

One thing i dont understand though is when we have a table like the one above, why do we have to work out $fx$ and $fx(squared)$ ? As when I go to do the standard deviation, this is not stated in the standard deviation equation.

Anyone? x

4. Originally Posted by leaaa
ok i understand now, I have to find the mid class values of the data first and work out the mean from there.

One thing i dont understand though is when we have a table like the one above, why do we have to work out $fx$ and $fx(squared)$ ? As when I go to do the standard deviation, this is not stated in the standard deviation equation.

Anyone? x
That is the definition of standard deviation. That takes into account all values will then be positive. The sum of these squares is the variance, but to get back to our average difference from the mean, or standard deviation, you need to take the square root of that sum.