Simple stats problem : Unable to get final answer

• October 26th 2008, 08:26 PM
tester85
Simple stats problem : Unable to get final answer
I have a simple stats problem where the following table is given.

Speed (Km/h)--------Frequency-----------Cumulative Frequency

0<s<=40--------------14----------------------14
40<s<=60-------------51----------------------65
60<s<=70-------------157---------------------222
70<s<=80-------------141---------------------363
80<s<=100------------37----------------------400

I am asked to find the median which i can find easily 51, but the standard deviation i tried all sorts of values but unable to get the given answer which is 64.6. I am getting like 13.67. Can someone guide me out on this issue ?
• October 26th 2008, 09:10 PM
CaptainBlack
Quote:

Originally Posted by tester85
I have a simple stats problem where the following table is given.

Speed (Km/h)--------Frequency-----------Cumulative Frequency

0<s<=40--------------14----------------------14
40<s<=60-------------51----------------------65
60<s<=70-------------157---------------------222
70<s<=80-------------141---------------------363
80<s<=100------------37----------------------400

I am asked to find the median which i can find easily 51, but the standard deviation i tried all sorts of values but unable to get the given answer which is 64.6. I am getting like 13.67. Can someone guide me out on this issue ?

The standard deviation cannot be 64.6.

The mean is:

$m=\frac{\sum_{i=1}^5 f_ix_i} {\sum_{i=1}^5 f_i}$

where $x_i$ is the cell mark and $f_i$ is the frequency of the $i$th cell respectivly.

then:

$s^2=\frac{\sum_{i=1}^5 f_i(x_i-m)^2}{\sum_{i=1}^5 f_i}$

is the variance and $s$ is the standard deviation.

CB
• October 26th 2008, 09:28 PM
tester85
I use a one of the standard deviation formula.

$\sum$ xifi = 26940 $\sum$ $xi^2$ fi = 1943530

$s^2$ = 1/400[ 1943530 - $(27340)^2$ /400 ] = 13.67

According to the formula stated i got 67.35 for the mean is that correct ?

May i know what is the value you got for the mean and standard deviation?