The unexplained variability is that within the treatments, i.e. differences between replicates. This variability is not caused by our experimental treatments hence is unexplained by our experiment. This is often called experimental error . The answer is experimental error , i.e. natural variability which is inevitable for most biological variables. The variability within a single treatment can be measured by calculating its SS , i.e. the SS for these four observations.
Similar explanations apply to the other treatments.
Thus, for each treatment we can calculate a SS . If we now sum these SS we will have a measure of unexplained variability. Since we now know the value of two elements in the summation we can find the third, the Explained SS , by subtraction. This is because the magnitude of a sum depends, to some extent, on the number of items summed. We can correct for this by dividing by the number of items in the summation, i.e. find the mean value.