Hi this seesion is about Arithmetic mean or simple average what really is arithmetic mean to understand this we need to look at the monthly rainfall figures of Colombo in the year 2012 this is the graph which depicts the monthly rainfall figures in 2012 right you need to find the average monthly rainfall in this particular year so how do you go about it Arithmetic mean or simple average is nothing but sum of all values upon the total number of values so what exactly needs to be done well as the formula suggested you need to add up all these 12 values right what’s the sum of all these 12 values if you add them the sum is 3000 and how many values have we added in all monthly right so in a year there are 12 months so we have added 12 values so the Arithmetic mean or simple average would be nothing but 3000 divided by 12 which is 250 in short in the year 2012 Colombo had an average monthly rainfall of 250 Milli-meters but you would have realize that in doing so it took a lot of time because we had to add so many big values isn’t there any other way around well that’s not the case we have an interesting method of finding Arithmetic mean which circumvents this difficult calculations what is that well that’s called the method of assumed mean what do you do in this method we assume the mean to be one of the values which is somewhere in between the highest and the lowest for example in this case the highest is 600 and the lowest is somewhere in the range of hundred for example this way so what we do is we assume the rainfall the average rainfall to be 200 well I am not suggesting that the actual average is 200 we are just assuming the value to be 200 now what we do is we find all the positive deviations which means the deviations of all the values which are more than 200 from 200 for example these are the values which are more than 200 and the deviations as you can see are 170, 350, 400, 120 and 20 deviations are nothing but the difference between the actual values and the assumed mean right likewise will go on to find out the negative deviation which means the deviations of those values which are less than the assumed mean these right so for example 130 is 70 less 150 is 50 less than you assume mean ow what is the advantage of doing this