Percent Deviation

Percent deviation problems bring the fact that the average of a set of measurements, such as weight, does not necessarily reflect the fact none of the data in the sample will be at the calculated average.
The percent deviation gives a number that indicates where the majority of measurements are. Percent deviation is most useful  in statistics and chemistry problems. It finds how far a measurement, on average, will deviate from the  mean value. This problem gives a set of measured  numerical data.
Deviation or Error Percent.

Percent deviation :
In order to find percent deviation first of all you need to find the Average deviation and Mean. After finding the two things you divide the average deviation into the mean then multiply by 100% . To Find the average deviation you need to subtract the mean from a measured value.  Mean value can be measured as sum the data values and then divide that number by the number of data values.
percent deviation = (Average Deviation/Mean) x 100

The following steps are required to find the percent deviation:
ex:  4, 5,9
Step 1: find the mean value of the measurements. Sum the data values and then divide that number by the number of data values
Mean =  (4+5+9)/3  = 18/3  = 6;
Step 2: Find the Average Deviation. Find the deviation of each  Data value, sum them and then divide by the number of measurements. The Deviation is calculated like as absolute values of subtraction of the each value from the mean.
Deviation = ( |4-6|+|5-6|+|9-6|);
= ( 2+1+3)
= 6
Average Deviation = 6/3 = 2  
Average Deviation = 2
Step 3: By using the Formula of percent deviation substitute the Average deviation and Mean then we get Percent deviation
Percent Deviation =  (Average Deviation/Mean) x 100
=  (2/6) x 100
= 33.33%

Exercise on Parent Deviation
1) A student  measured the length of his  desk as  81cm.Later he found  that it was actually 75cm long. What was his percent deviation ?
2) A student the volume of  a piece of rock to be 25 grams and the accepted value is 19 grams. What is the deviation in percentage?