Dinah YoungProject Manager / Software Asset Manager| Prince William CountySpringfield, Va, United States
I am usually really good with the calculations and such from the PMI tests. But one thing I have never quite understood is standard deviation. What is it, what is it supposed to represent, what is it used for. I see a bell curve and 1-3 Sigmas. But I am just not understanding. Can someone explain it to me in simple terms? Saving Changes...
Zeeshan AmjadCoach| Kimza LLCAshburn, Va, United States
Good real life example Mounir, but with one little problem. The example is actually for median not for mean. If you need 50% on the left and 50% data on the right then it is actually median.
Let me give try to explain. Supposed you commute 10 days with the following commute time
30, 38, 35, 35, 37, 96, 35, 33, 34, 38
As you can see there is an out liar on day 6, may be an accident on that day and it took 96 minutes to reach the office on that day.
Now the mean of this is 41 and median of this is 35. If you take a look at the mean which is 41, then there is only one value greater than 41 and rest of the values are less than that.
However in case of median, 50% values are smaller than 35 as well as 50% values are greater than this.
Median is a great tool to remove the out liar in the data. For this reason usually median salary and median property value is reported to avoid any skewness because of one or more out lairs.
You can always calculate the higher moment, such as skewness (3rd moment) or kurtosis (4th moment) to get more insight about the data, but it is more calculation and well beyond the PMP outline.
The standard deviation tells us how scattered the data are with respect to the mean. When there is extreme data, the standard deviation usually increases. Not many people use it. Whenever you use measures of central tendency, include the standard deviation Saving Changes...
